Nick here from Shopify.
Really good question. When you sell with Shopify POS there is no commission to Shopify, the only fees which would be incurred are the standard credit card fees. If you are using Shopify Payments as your Payment Gateway you can see what the credit card fees for each Shopify Plan in the image below.
If you are using a payment gateway other than Shopify Payments you would need to find out from the gateway directly about what their credit card fees would be as each gateway would charge a different amount.
I hope that helps and answers your question.
All the best, Nick