When a non-US-issued card is used in a US merchant account, two extra fees apply on top of normal interchange: an international interchange rate (typically 30–50 basis points higher than the equivalent US card) and a cross-border fee (about 0.4%–1.0% from Visa/Mastercard, depending on whether the transaction is in USD or the card's local currency).
For e-commerce merchants with significant international traffic, this means international transactions can cost 4%+ effective rate vs 2.5%–3% for domestic. Strategies to reduce: present prices in USD only (avoids dynamic currency conversion fees), accept local payment methods (iDEAL in Netherlands, Bancontact in Belgium, etc.), or use a global processor like Adyen that has local acquiring relationships in major markets.