camila.254

I use this module: https://addons.prestashop.com/en/other-payment-methods/28289-payment-with-fee-paypal-stripe-cod-bank-wire-etc.html It allows to add a fee amount (by percentage or a fixed a mount) to official Paypal module as well as any other payment modules of Prestashop. You can try it.

looks like it's problem of the custom theme You can try to switch to default theme and see if the problem still happens

this problem can only happen if you install some custom module that making changes to the order process. Try to check your module list and disable custom modules that you think it can be problem... then test the order process again

you can only set shipping cost by weight ranges for general shopping cart. In your case I would suggest you to add also weigh to combination to define shipping cost (if you have fixed weight values)

This problem happens because you're using a custom theme that contains the bug when developer made changes to product.tpl files. You should contact the developer and ask for a better version with bug fixes