All Shopify app payments are run through Shopify, meaning Pixelpop will appear as an item on your monthly Shopify bill in whatever currency you have set up. You'll pay for Pixelpop however you pay for Shopify.
For BigCommerce, you'll be prompted to enter your credit card details on the Account page when you choose your plan. We accept Visa, Mastercard and American Express. All payments will be processed in US Dollars plus applicable taxes.