I'm Miles from the Social Care team at Shopify.
As PayPal sets their own fees, so for an answer on what you'd be charged from them, you'll need to contact PayPal directly. In regards to what Shopify will charge, that will depend on if you're also using Shopify Payments or not.
Essentially, if you're using Shopify Payments along with any other gateways, you'll only be charged the fee set by the gateway that the customer used. For example, if you have Shopify Payments, PayPal and Sezzle set up, and the customer paid via PayPal, you'd only be charged PayPal rates. Again, if they paid using Sezzle, you'd only pay fees to Sezzle. Depending on what plan you're on, you'll be charged a fee for using Shopify Payments, if that's how the customer paid. For information on what fees are charged for these transactions, check out our pricing guide, and check the fees for your plan.
If you're not using Shopify Payments at all, then you would be charged the third-party's fees, plus a fee from Shopify. Again, this depends on what plan you're on, so check out the pricing guide linked above. These fees range from 0.5% to 2%.
I hope that help to clarify the fees associated with Shopify Payments and other gateways. I'd also recommend checking out our guide to charges with Shopify as well: https://bit.ly/2W4kHz6.
If you have any questions at all, please don't hesitate to reply to this thread.