Q: I'm in California but since Printex is in Missouri, do I need to charge tax in California?
A: Disclaimer: This is NOT tax advice.
This article explains how you interact with your customer and offers some links to useful articles. If you have further questions about collecting sales tax, please consult an accountant, as laws vary by state.
The way the T-REX ecommerce works is:
1) A customer purchases a product from you.
2) You purchase the product (wholesale or discount) from a supplier (ie. Printex, other print on demand company)
3) The supplier prints your product and/or fulfills your order and ships to your customer on your behalf.
Printex does not exchange money with your customer.
Since you collect money from your customer, it is your responsibility to collect tax (if applicable) per your state's laws.
Here are a couple of useful articles to assist you with your research.