I'm sure that you're already past your bank's midnight deadline for returning the checks, so don't even consider it.
First, you need affidavits by the payees of the checks to the effect that they did not receive, endorse, deposit or obtain the benefit of funds from the checks. It's your customer's job to get those affidavits for you from his payees, since they become the basis of your customer's claim for reimbursement from you.
You take each check and send it with its affidavit to the depositary bank directly, without entry. You include a cover letter explaining that you're entering a claim under the depositary bank's transfer and presentment warranties of the endorsements under the UCC.
If the depositary bank accepts your claim, it will send you a cashier's check to satisfy the claim.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8