Any tax liability you are told you may have can only refer to your own INCOME TAX. Its not written very clear. That is where the confusion seems to be. I am sure the buyers are aware of sales tax for their state at checkout. The .65c Redbubble taxes I would say are taxes they paid on their own costs which you got a little back on seeing as a business they can claim back on certain expenses. (As said above: the base price + RB's tax 10.95 x 35.9%) = 3.93
But regarding SALES TAX, there are still about 5 states that dont have any, but this is from RedBubble's page from the BUYER'S point of view:
US Sales Tax
What is the tax? The tax is a sales-based transaction tax on online sales, similar to sales made from physical stores.
How do I pay my sales tax? Check out with your items as usual. The full tax amount will be included in the purchase.
How is it calculated? The tax rate applied to your order will be the combined state and local rates of the address to where your order is delivered. It is calculated after the shipping address for your order is completed. For example, if you live in a state that does not impose a sales tax, you may still see the tax calculated on your order if shipped to another state that does.
What is my tax rate? Taxes vary by States and Zip Codes. Please refer to your local state tax laws for further information.
page here: https://help.redbubble.com/hc/en-us/articles/360018700712
Items sold on Redbubble.com and shipped to locations in the US, including territories, may be subject to sales tax. Based on applicable tax laws, Redbubble will calculate, collect, and remit sales tax for orders shipped to customers in the following state(s): Alabama, Arkansas, California, Colorado, Connecticut, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, Washington, Washington DC, West Virginia, Wisconsin, Wyoming, Utah