I am impressed getting to know that some POS now accepts Bitcoin as a means of payment in exchange for fiat note in Nigeria, and as an experienced person who has done many physical BTC/Fiat transaction with people, I will love to advise that there will always be challenges in regards to delay in confirmation of Bitcoin transaction, but it always goes through in worst case scenario, but in a case whereby a customer claims to have sent BTC and not yet reflecting in your wallet, all you have to do is check it on Blockchain.com/BTC explorer using your wallet address, as it will show an overview of all transactions both confirmed and unconfirmed transactions
Sending each blockchain network actually get delay with bitcoin about 30 minutes until several hours later depending how busy with blockchain confirmation, I have little ideas for OP maybe could adopt and use all exchange market global with his costumer could sending trough lightening network or sending by ID of exchange market account. All costumer not happy with delay transaction exactly what problem faced by OP because need to clear confirmation waiting until 35 minutes later for delay. By adapting send bitcoin assets trough exchange market become faster and instant without few second transaction have been arrived on the destination wallet sender.
Not have reason how to manipulated with blockchain transaction exactly when sending bitcoin, OP can see with transaction history and open blockchain to know how many confirmation need before transaction success.