Thanks for asking your question! I'm sorry to hear about your tax issue and I'm going to try my best to help you understand or resolve it.
Thank you for your question, and thanks for using JustAnswer.com. Your sales would be subject to sales tax (which you collect from the company) for customers inside the same state as the drop shipper. So, if your goods are shipping out of a warehouse in California, all customers in California must pay sales tax.
Since your business is registered in South Dakota, you would be subject to income tax within the United States. I see you have an LLC and an EIN, an EIN is a tax number and is what you would use on your tax filings. An EIN can be obtained by filling out form SS-4 available, HERE
Please let me know if you need anything additional. Thanks again for using JustAnswer.com and have a great day.
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You should be aware that your question, as with most tax questions, can never really be answered completely...addressing all the permutations; that is because of the many assumptions that have to be made I have done my best to determine what I think you are asking and answering it in the most direct and understandable manner possible. If, however, after reviewing the questions, you have any uncertainties or further questions, please do not hesitate to ask.