This is required by law.
I do not know why i have this value and the tax amount in the database,
but the payment percent and the number of days are hardcoded. I guess i
am such an inconsistent mess.
I have to basically do the reverse of signing the request to verify that
the notification comes from them. Lots of code just for that.
I return the changed status from the PL/pgSQL function because i will
need to email customers when a payment is completed, and i need to know
when.