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In the EU and (for now) UK the interchange rate is capped at 0.3% cc and 0.2% dc for domestic transactions (there are different rates for cross border depending on which countries are involved)...

Check the Open Cart link from my post above - it has the fees for Paypal Commerce listed.

I should have clarified that is for credit/debit card transactions. Paypal transactions are the standard 2.9%+30p

Open Cart and Woocommerce are two in the Paypal directory.
https://www.opencart.com/index.php?route=marketplace/extension/info&extension_id=38358
...

Paypal sales rep. 1.2% + 30p per transaction and no monthly fee.

Unfortunately as Zen Cart is US centric and 3DS isn't yet required for US credit card payments there does not seem to be any urgency from the devs to switch from WPP to Paypal Commerce.

The Square...

I'm hoping you have better luck getting an answer than I did
https://www.zen-cart.com/showthread.php?228589-Upgrade-alternative-to-WPPro-for-EMV-3DS-2-0

Currently my WPP checkout is still working...