Suppose that I send invoices to my clients recurrently. Sometimes the amount is too small to pay alone. In that case, I don't mind they pay with next invoice.
You can pay it together with next one.
Is that polite? As for me, it sounds as if I am a boss and give them some permission. That is not my intention.
I usually communicate with some clients on email and others on text chat. So I'd like to know suitable expressions for each case. (maybe chat is less formal than email)