Mouser has EU warehouses, so you shouldn't need to pay any customs. However, you will be paying VAT. The same as if you were ordering or buying something from a local store.
Digikey ships everything from the US, they don't have any EU stock, despite having EU phone numbers on the website and trying to look like an EU company. The "customs" are usually collected by UPS or who they use today because they charge a flat customs declaration fee (about 20 euro) on everything, regardless whether or not the order actually has any customs due. So you are most likely paying that - the rip-off paperwork fee and not the actual customs. If you make fuss with Digikey, you can often get this ridiculous fee waived/reimbursed.
You likely won't need to pay VAT for Digikey orders because your order is most likely below the VAT collection threshold on import (the paperwork to collect VAT from a few resistors would cost 10x more than the tax collected).
However, be aware that this varies between EU countries, because the tax and customs legislation is very different from country to country and not harmonized across the EU (tax regs were explicitly left to the individual member states). E.g. in Denmark I didn't have to pay any fees when ordering from Digikey. Here in France if I order from there now, I get slapped with that 20 euro paperwork fee. Elsewhere you get both the papework fee AND VAT because that country charges VAT on every import (or have some ridiculously low threshold like 20 EUR for it).
This has been discussed on the forum in the past.