According to the help page, you can pay by card (Visa, Mastercard or American Express), with Apple Pay or Google Pay, or through PayPal. We could not find cash on delivery or installment plans mentioned anywhere on their pages, so don't rely on either.
At the time of our check, the site was promoting an extra first-order discount that came with a free gift product, a separate extra discount available only in the app, plus a Refer a Friend scheme that gives you a reward once a friend you invited places an order. Offers like these change frequently, so glance at the site's top banner before you order.
Source: Myprotein: help centre