The Instant Access fee is based on the receivables purchased from your marketplace. Receivables include gross sales as well as, Amazon Account level Reserve balances when applicable.
You can review your Payability fees on your dashboard under Reports > Fee Report or on the Daily Summary
In the example below, you can see that we purchased $1,066.14 in receivables from the marketplace. The 2.5% fee on $1,066.14 was $26.65. The fee is deducted directly from the available balance each day.