Every referral, closing, FBA and shipping fee Amazon deducts from your payout carries 18 percent
Amazon charges 18 percent GST on referral, closing, FBA, weight handling, storage and shipping fees, and issues tax invoices for them in your name under Reports and Tax Document
The GST on those invoices is input tax credit under Section 16, claimable in GSTR-3B Table 4A(5) once the invoice appears in your GSTR-2B.
You may receive invoices from more than one Amazon entity, each with its own GSTIN, and each must be matched to GSTR-2B separately.
Amazon's credit notes for fee reversals reduce the credit you can claim, and TCS is never ITC; it goes to the cash ledger instead.
Open Reports, then Tax Document Library, then the seller fee invoices section. Invoices are listed by month and by issuing Amazon entity.
The full guide has the formats, the deadlines and the worked examples.