In the sentence you have a relative clause introduced by que which refers to something which may or may not happen in the future [al final será] i.e that the consumers will pay the invoice, therefore we use the subjunctive.

The present tense would only work if you 100% knew who the customers were, that they existed and you knew they were definitely going to pay in the future.

The imperative would not work in this situation as it is used to give orders, advice or make requests and suggestions.

Relative clauses using que come up in the B2 lessons.

I suspect that Nicole is unwittingly using the word "imperative" - she meant to say or should have said "indicative"... Just as Marsha implies, the subjunctive tends to be used with situations which could be described as vague and undefined. Thus, a sentence expressed as a negative will sometimes be rendered in the subjunctive when the corresponding positive form will be in the indicative. Guidelines as to when to use the subjunctive run to many pages. Opinions and personal judgements usually fall into that category. Another useful one is that "verbs of influence" tend to take the the subjunctive; e.g. ordering, persuading, recommending, preventing, forbidding - and even hoping and wishing [as in the lesson Nicole is referring to].

Wow! I am so grateful to all of you for your helpful replies. Thank you Marsh, David and Inma.

Unfortunately, my confusion does have to do with the "Imperative", and those indirect orders. It could be a confusion from the non-Spanish grammar side of things.

For example:

If I were to say : "You need to clean your room."

Is that in the  imperative (indirect imperative  -if there is such a thing -) 

or would that translate to be in the subjunctive in Spanish?

I put that sentence in Reverso and got this:

If I say: I need you to clean your room = Necesito que limpies tu cuarto (Subj)

You need to clean your room:  Tienes que limpiar tu habitación.

(When I say :"you need to..." does that not imply that "I" am saying that "he/she needs to clean their room"?

I think part of my problem might be solved in this lesson:

Forming simple negative commands : No + El Presente de Subjuntivo (present subjunctive)

but when I tried to open it, it said it was not available at this time ...

This is the message: "Oops! That item is not currently available"

Also, I believe part of my confusion comes from being told the following:

¡No estudies ahora! - negative command, formed with subjunctive" "

which lead me to think that sentences like this are in the subjunctive mood and not the imperative.

Maybe from all this you spot my confusion.... and what I'm missing.

Sorry for all the confusion, luckily, you're not inside my head :)

Thank you.

Nicole

Erratum:  Just as a quick note - when I said the "present" in my closing sentence in my original question, I means Present indicative: 

"I would have used the present or imperative here more so. (Would it be possible?)

 Sorry about that.  

Nicole

Hello agan Nicole -

1. The sentence "Tienes que limpiar la casa" is indeed a recommendation or instruction, but it is not being expressed in the 'Imperative Mood' (nor with the subjunctive); it can be managed with a simple construction consisting just of the present tense plus an infinitive...

... "Es necesario limpiar la casa" is also permitted, I believe, but if you decided that something more emphatic and insistent was appropriate, you could say "Es necesario que limpies tu casa" - which is in the subjunctive because it can be regarded as coming under a 'verb of influence', almost like a recommendation/command (as well as containing overtones of 'judgement' by the person making the comment on the state of your house !)

2. Yes, a negative "tu" command does indeed look like a subjunctive, but it might be better just to regard that as an 'aid' or a guideline as to how to compose it...  An expert or specialist in the history of the Spanish language might well be able to explain how and why imperatives [including many affirmative ones] became based on the subjunctive [and perhaps *when* that happened, during the development of early or mediaeval Spanish out of Late Latin] - but this is more likely to be something 'of interest' [to certain people] rather than a way of grasping the character of the Spanish subjunctive... I confess that I am intrigued by it, and will try and make inquiries. 

I am probably approaching the limit of my abilities here: i.e., it could be asked whether I really am entitled to try and help - (I am not a native speaker, nor a trained teacher !)

Good luck with your studies !

Hello. 

Thank you David, you certainly could have fooled me, in my book you qualify as a top-notch "teacher" and can be my teacher any day!  :)

Inma,

Thank you for your great explanations and references.  I will study these and practice as well.

I really pondered where my confusion comes from, and if I have this right, it is only the "same spelling" as the subjunctive that is used and not that "magically" that this "mood" has transformed from the "Imperative" to the "Subjunctive.  There may be other "hidden" info, however....

Also, in English, a "request" is an order = imperative (from what I found online)

In Spanish it is couched in the Subjunctive. 

I believe that in English we make things more "polite" in using words like "could you, would you, please etc" while in Spanish all that is expressed with the Subjunctive.

I really love how Spanish has this ability to convey so much with so little! It's quite amazing. 

Thanking you all again for your help and understanding and taking the time to help.  It means the world to me.  Have a great day!

Nicole

Thanks Nicole ... I noticed that your question in "Subjunctive or conditional", posted on 18th April, has not yet been answered, so I tried adding a few thoughts and comments: (admittedly, maybe by now you have already resolved your doubts !)

Hi Nicole, I wondered about this too, as some instructors call the "Qué" statements "indirect commands".  So, is the tense that follows the "qué" the imperative or the subjunctive?  Then I noticed that these statements never use the "tú" imperative conjugation, such as "ve" or "haz".  Also these statements are expressed in indirect form (we don't say "Qué usted vaya bien" but "Qué le vaya bien").  So it was refreshing to see Inma call these statements wishes and not commands, which is consistent with using the subjunctive.

Marcos,

You refer to "Qué" statements - but I am wondering if you really mean "Que" statements? - [i.e., removing the accent].

Ha, good point David.