Simple.
Just ask about where you can get switches, too. They might just send keycaps AND switches
Better yet, also ask if they have some sort of tutorial/guide on how to replace a broken switch (you could add some nifty line to the tune off "since being able to change a single broken switch is one of the key selling points of mechs, I am sure there has to be a way").
They might just offer you to send it in and do the fix for you.
I read Forum articles in the Deck forum where folks had this situation and they just had to pay for the postal service.
One guy however had to pay for the full repair because according to Deck his keyboard was full of ashes and "debris" from smoking, which voids the warranty according to Deck