There is no fixed percentage. Amazon only collects the amount that is due to Uncapped on the date of your disbursement not a set share of your disbursement. This stays the same for the life of the loan: it's always based on the dollar amount due, never a percentage.
Here's an illustrative example, assuming your repayment to Uncapped is $100/day and we collect only the due portion on each Amazon disbursement.
If the repayment amount due is larger than a given disbursement, we collect the shortfall on the next one.