Starting at the end of March, and at the end of every month after that, I would pay you $50
In exchange for you receiving the $50, you would agree and complete any minor bug fixes that might arise during the month, provided the work involved wouldn't take more than 2-3 hours.
Eg. I identify a bug fix on 10 March and contact you.....you look at the issue and confirm it will take 1 hoyr to fix it.....I agree and then you fix the problem.....at the end of the month you get the $50.
If the problem would take more than 2-3 hours to fix you provide me with a price for the additional time beyond 3 hours and I pay you that amount immediately after the work is completed then pay you the usual $50 at the end of the month.
If there are no bug fixes required in the month, you still get the $50 and may get $50 per month for several months without ever having to do anything.
The reason for making the payment at the end of the month is to ensure a prompt response to any buigs that might be identified.