I note that https://www.paypal-community.com/t5/Business-Tools/One-time-purchase-and-recurring-payments-in-single-transaction/td-p/1509420 has a similar question and has an answer; I suspect the same author may have asked both questions, but thought this could be useful for others seeking an answer.
To avoid link rot, I'll paste the answer given there below - but I haven't personally tried this so I do not know if it worked.
One option for a one time payment is to use the Trial Variables set
for one day, after that the regular payment starts on the second day.
For example, you run a service business:
There's a Onetime Setup Fee of $25.00 plus a Monthly Maintenance Fee
of $10.00. The Setup Fee is charged by using the Trial Variables set
for one day, after that the regular payment starts on the second day.
Some sample code with notes:
<!-- "a1" Trial amount 1. This is the price of the first trial period.
For a free trial, use a value of 0 -->
<input type="hidden" name="a1" value="25.00">
<!-- "p1" Trial period 1. This is the length of the first trial period.
The number is modified by the trial period 1 units (t1, below) -->
<input type="hidden" name="p1" value="1">
<!-- "t1" Trial period 1 units. This is the units of trial period 1 (p1, above).
Acceptable values are: D (days), W (weeks), M (months), Y (years) -->
<input type="hidden" name="t1" value="D">
Not sure if this is what you have in mind but, it may give you some other ideas too.