Honestly, if you’re on WordPress, you don’t need a separate billing platform just grab a plugin. For most stores, WooCommerce (with the official 2Checkout and PayPal gateways and the Subscriptions add‑on if you need recurring billing) handles everything on‑site. If you’re in hosting and want a full‑blown client portal, WHMCS (via the WHMCS Bridge plugin) or Blesta both integrate nicely so users never leave your WP theme. But for straightforward invoicing/payments, something like Sprout Invoices or WP‑Invoice will let you bill directly from WordPress without redirects.