I assume you've worked through the section on Washington's B&O tax to determine whether you're subject to that tax, esp. the "service and other business activities" tax under WAC 458-20-224. The issue could potentially come up for Hawaii, New Mexico, South Dakota and Delaware. In SD, there is a 4.5% tax on enumerated services most of which cannot be provided except in person, but it also includes
membership or entrance fees for the use of a facility or for the right
to purchase tangible personal property, any product transferred
electronically, or services
Sec. 10-45-4 imposes
a tax at the same rate as that imposed upon sales of tangible personal
property in this state upon the gross receipts of any person from the
engaging or continuing in the practice of any business in which a
service is rendered
and that state rate is currently 4.5% (there are also municipal taxes). Because of South Dakota v. Wayfair, your sales to SD customers could be taxed. The relevant issue is whether there is a "substantial nexus" with that state, as required by Complete Auto v. Brady. The SD law defines the threshold for that level of business as $100,000 of 200 transactions, annually. It is likely that your business will remain "under the radar", but the court was not explicit about how low the radar can go.