Payment options are mada, Visa or Mastercard cards, Apple Pay, gift card balance, and cash on delivery. Paying by card at the door is not offered.
Instalments come through Tabby or Tamara, both of which take orders worth SAR 200 up to 20,000.
The payment methods page caps cash on delivery at SAR 2,500, and the shipping policy adds a SAR 20 charge for any order up to SAR 1,500. Paying upfront avoids that cash charge, and if you send something back later you can pick a refund to your card or to your balance.
Source: Under Armour Saudi Arabia: payment methods