Vinted USA: Question about shipping with SpeedX

So, I opened a vinted account about 1-2 months ago. I have only sold like 6 items (I'm more active on depop). Anyways...I always ship with USPS and had other shipping options turned off. Today I sold an item and for some reason, the shipping was through SpeedX (first time hearing of this company btw). I went to my shipping options and turned it off again. I have no idea why it was turned on to begin with.

The thing is I can't find any drop off locations near me. I'm thinking of refunding the buyer and letting them know the reason why. However, I don't know how to issue a refund. I do see the option of cancelling the order. It would be my first time having to do this so I want to make sure I don't do anything wrong