I recently added a location in anticipation for an upcoming craft fair, so I can charge the proper tax rate at that site. when I look at my POS app I see that all my inventory is only available at my original online store location. How do I sync to have all my inventory always with me, whether at home or at a craft fair? If I add numbers to my fair location it will double my counts, which isn't correct. I cannot pay for another add-on app at this time.
If you expect to only be only selling at a few craft shows, it may be worth the small hassle to use an iPad (assuming it's 3G+ or there is wi-fi at the event) to process orders. This is particularly true if you make more one-of-a-kind inventory -- without having your inventory quantity synced from Shopify, you risk overselling items.
If your primary concern is the ability to accept payments outside of Shopify, in addition to the apps Batal Games pointed out, check out Spark Pay and Vend -- or even Square (though it cannot be integrated with Shopify).
Stitch integrates with Spark Pay, so you can manage both online (Shopify) and offline (craft show) transactions all in one place, as well as understand what sells best through each channel.
My concern isn't the method of payment, but rather how to have my inventory show up online at the craft fair location. After posting this question I discovered the way to virtually transfer my inventory to the offsite location, which is what I plan to do the day before, then transfer the remaining goods back home at the end.