As others have already said here: Yes, USPS First class parcel post international is the cheapest by far if you sell light (under 13 oz) items. Never, never go through UPS or FedEx, always go to your local Post Office!
My question is: Why did you not look online first? All of these places - USPS, Fed Ex and UPS have postage calculators on their website. I had basically the same experience as you, I got a purchase from someone in Sweden and looking at the UPS postal calculator gave me sticker shock, it was about what you said - $37. I found out almost right away though, that that was a rip-off by going to the USPS website.
You could've known last night, before bothering to drive to a UPS store that it was not your best bet. Always check online first!