Card processors (Stripe in our case) charge 2.9% plus a flat $0.30 per transaction. The flat part doesn't shrink as your purchase grows, so a small buy looks fee-heavy compared to a larger one even though the same flat cost is being applied.
💡 Buying more at once reduces the fee per square. At 1 square the fee is $0.33 per square; at 50 squares it drops to about $0.02 per square.