I can't go first, so we will need to use an escrow. PM me if you want.
After escrow confirms payment, I will send a code which can be entered in the Steam website to redeem your credits.
Buy 3 and get 5% off the total price.
Buy 5 and get 10% off the total price.
I'm just saying Steam Gift cards for 2% off is literally nothing...
Even supermarkets sometimes have them at 10% off, or even more.
I would reconsider your pricing if you want to stay competitive. At least 10% off is a good starting point, then if someone buys 3 then give them 15%, if they buy 5 then give them 17.5% off, so on so forth.
Plus, do you have any sort of proof of purchase for these codes?
Steam codes can be carded since game keys can be sold for bitcoins easily... A receipt or something like that will do.