If not, will request that the buyer pays again and I will send back his first 2 transactions in case they get confirmed later, which I seriously doubt.
From what I've read so far it seems like you are not using a payment processor to process your deal (CMIIW). I suggest you look for an open source payment processor so you don't have to manually check the confirmation and the buyer would likely get incentivized to pay a reasonable fee since their deal won't get processed if they failed to send a confirmed transaction within a time frame. You should spend some time browsing this forum with keyword like "payment processor", you'll definitely learn a lot and knows what to avoid as a seller. Good luck.