TLDR: What would be the chances of performing a successful Replace By Fee (double spend) before the block gets confirmed?
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How likely are RBF successful? Is there any way I can prevent this?
if the transaction has a very low fee at a time of mempool size is considerably big (= congested network)
then the chance to perform successful RBF is pretty good
prevention: by requiring confirmation on that bitcoin transaction before letting customer leaves your store with the product
My concern is: what if a customer comes and buys a MacBook or any product, completes the purchase with Bitcoin, then leaves the store and re-broadcasts a transaction with slightly higher fee back to themselves within the next 10 minutes.
when you say
"completes the purchase with Bitcoin", the transaction should have at least 1 confirmation
if it hasn't received any confirmation yet, you shouldn't handover the product to your customer
think of it as if you are accepting payment with debit/credit card,
the payment is consider done when you get confirmation from card issuer thru EDC device
in bitcoin peer-to-peer payment system, you get confirmation from bitcoin nodes in the network