The answer may vary depending on your state. If you're in a state that's a member of the Streamlined Sales and Use Tax Agreement,* there's a designated heirarchy for sourcing sales of digital goods:
First, if you're making delivery to the customer at your location, source to the location where you make the sale. If not, source to the location where your customer will receive the product.
Neither of those works for you, so you'd continue down the list to the first one you can apply:
- The purchaser’s address that you maintain in the ordinary course of the your business;
- The purchaser's address obtained during the consummation of the sale;
- The address where you first make the product available for transmission or the address from which you provided the service.
By my reading, that means that in the absence of an address, you basically come back full circle and source the sale back to your own location.
*Arkansas, Georgia, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Nebraska, Nevada, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Rhode Island, South Dakota, Tennessee, Utah, Vermont, Washington, West Virginia, Wisconsin and Wyoming