The only way to really do this with standard paypal buttons and shopify plans is by baking it into product prices.
If you know in advance they are going to use paypal before the checkout flow then you could just do some advanced customizations and use two sets of inventory one for normal one for paypal checkouts.
If you want to set this on the product page you could add paypal as a variant option and then only let paypal buttons show when that variant is selected this way also you can bake the words 'paypal fee', etc into the product title to remove ambiguity about why the price is different.
Or add a companion product to the cart.
Remember messaging - Big thing to be careful of though when swapping product&prices is making sure the customer is aware this has happened or it can seem like a bait and switch which is super bad. So using a companion product as a fee can be much better.
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