(I'm rating the Vx670 as a credit card terminal, not the seller).
My wife does arts and craft shows. For the last 4 years, we've used a Vx670 Wifi terminal (and a portable Mifi service to bring our own private and secure Wifi service with us). The Vx670 "just works" all the time. It is PCI compliant and your service provider should be able to update and program it for you (or you need a new service provider).
If you sell arts and crafts, you really should accept credit cards and you want something that works well when you get busy. The Vx670 is up to the task. Out in the middle of nowhere, as long as your MiFi service (use Verizon or Sprint) has signal, bring a 12volt battery and you can run this terminal and your MiFi for days without recharging.
For low cost and reliable MiFi, take a look at [...] ... they offer 3G dataplans starting around $10/month and if you are ONLY doing credit card transactions with your Vx670, you don't need very much monthly download allowance.