Payment processing fees
Every time a customer pays with a credit card, you pay a processing fee. With Shopify Payments (Shopify's built-in processor), these range from 2.4% to 2.9% + $0.30 per transaction depending on your plan.
The higher your plan level, the lower your rate. This is meaningful at scale: on $50,000 monthly revenue, the 0.5% difference between Basic and Advanced saves $250/month.