adding a verification check to registration

I would like to know if a) anyone has already done this, or b) if someone will do it for me?

Some background: I’m using this plugin to take payment for a four course certification program for a two-year certification. The client must take each of the four courses over a two year period, within 4-8 months of each other. No sooner, no later. If they wait too long, they must re-certify, starting with the first course again. If they try it too soon, they are gently rejected and told to wait until the 4-month rest is over.

What I need is something that interrupts the registration process to ask the reader if they have completed a prerequisite(s) for the course, and when that was completed.

If they have not taken it, or if it was more than 8 months ago, they need to be told to sign up for the first course instead of the one they’re trying to purchase, and should NOT be allowed to continue the purchase of that course.

If they have taken the prerequisite and fall within the 4/8 month window, then they simply continue to the checkout process.

I hope this is enough info to explain what I’m trying to do, and I hope this doesn’t describe a nightmare of coding. I can pluck at the low-hanging fruit in PHP, but the higher stuff is beyond me.

Thanks in advance,
C

https://www.ads-software.com/plugins/paid-memberships-pro-for-wp-courseware/