I am contemplating alternative or additional business/ payments structure though and would like to enquire if those are permissible or not: Will it be

Question

I am contemplating alternative or additional business/ payments structure though and would like to enquire if those are permissible or not: Will it be permissible to charge my clients a subscription fee (monthly or annual) which includes a set number of transactions service that I would have to fulfil on their behalf? However, it may so happen that they decide not to use my service in full or at all in any given month. In that case, what happens? Should I issue a full refund of the already paid subscription? If, for any reason, I am unable to fulfil all the transactions that are included in their subscription plan, are they entitled to a full or partial refund?