This is from another person who tried it and from whom I learned the trick with the 90 days, hopefully this helps to improve this plugin:
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I had the same error. I found information that helped me understand the problem and what to do about it.
Using days for the unit of time is awkward for the subscription.
A3 = Price
P3 = Subscription Duration
T3 = Subscription Duration Units
Originally Posted by PayPal
D – for days; allowable range for p3 is 1 to 90
W – for weeks; allowable range for p3 is 1 to 52
M – for months; allowable range for p3 is 1 to 24
Y – for years; allowable range for p3 is 1 to 5
If you want to use recurring payments with days as the duration unit, then the maximum that will work with paypal is 90 days.
If you want a recurring annual subscription, then the unit of measure needs to be months or years.
So for my site, an annual payment of $19.95 for 365 days worked, but if I made it recurring you get the error message above. If I want it to be recurring, I'll have to invoice it manually.
I also added a 90 day recurring subscription for $5.95. So for those who choose that, then payments will be made till they stop them.
I need to decide if I am going to offer both options or restrict it to a 90 day recurring payment only.
I would like to see an monthly or annual option in addition to or instead of days.
I am glad to have the script, except for this one thing, it seems to work well.