Saw some confusion in places about this so I wanted to clarify. At least in the US, Paypal does provide their "Pay in 4" short-term credit-free loan option for buying the Steam Deck. This allows Paypal to pay Steam for your SD, and then you pay Paypal 4 times, once every two weeks, no interest.

If you want to do this I highly recommend going through Steam Checkout on your Desktop PC. For some reason Paypal sometimes won't show the "Pay in 4" option on mobile phones, so it's best not to risk it.

Either through your Desktop Browser, or through Steam Desktop client, go through the checkout and choose the "Paypal" payment potion(not the My Paypal saved option if you've used Paypal before) and make sure you uncheck the box to Save the payment method. When you continue you click the Paypal button and then your browser will redirect you or open a pop-up where you can login to Paypal. Once Paypal asks you to pick a payment option, scroll down to the Pay later options and choose "Pay in 4". Continue and Paypal will ask to confirm a few things as well as what payment method to setup automatic payments with, and then Paypal will send you back to Steam so you can finish the checkout there.

That's all you need to do.

I chose this method myself, primarily because of the reliability. Since Paypal is paying Steam directly themselves, I didn't have to worry about getting snagged by my Bank's or CC company's fraud protection and having that screw up my SD order. I safely got my Steam Deck ordered with no fuss, and then I just had to pay Paypal afterwards, and in smaller amounts too, so I have less to worry about with my Bank or CC.

Edit: Added some emphasis