I use what is called an "evergreen clause" in my service agreements.

This requires that my client maintain a minimum balance in a retainer account to be billed against each month.
Example #1:

1) Client signs agreement and pays $2,500 as a retainer.

2) At the end of the first month I issue an invoice of $1,200 for services provided.

3) That invoice will be paid by drawing $1,200 from the $2,500 retainer invoice.

4) The client's retainer balance is now below the $2,500 minimum requirement, so . . .

5) I need to be able to issue a payment request to the client for $1,200 to "refill the till," and apply the client's payment to the retainer invoice.

OR, if that is not possible (Which is what I need)

6) I need to collect another $1,200 and issue a second retainer invoice,

If I can't do #5 and have to do #6, the client now has 2 retainer invoices. The 1st retainer invoice with $1,300 available to bill against and a 2nd retainer invoice with $1,200 available to bill against.