UPS does not require a signature on all items over $200, only if the shipper requests so. If the 9th & 10th digits of your tracking # are " 03 " UPS can leave it without any contact of you. They should leave it somewhere safe where it won't get stolen.
Are there streets close to you with your address?? Like 123 Tree Ln and your street is next one over 123 Leaf St, sometimes the driver could think he is a one street when he actually left it one block over, still a mistake on his part. Might be worth a quick drive around your neighborhood to see if you see a large box sitting on someone else's porch area with your name and address on it.
You already contatcted the board shop, so now UPS needs to get your signature that you received the boards which you didn't. The claim will be between the shipper and UPS, ask the board seller if they can send you new ones and request a signature so they know you will get them this time.
I have gotten stuff from both companies and have had good/bad with both, mainly issues with FedEx