Oh, no, that IS terrible!
Please remember, when you go back to Joann's, that it is not the fault of the employees that you speak to -- they are just cogs in a machine. Cold, steely, controlled anger and persistence is the best way to communicate. Ask to speak to the store manager, and go back if necessary to find that person in. Sorry to preach, but I cringe for mere sales clerks who are the subject of someone's anger.
One suggestion: Try to find a lightweight iron on stabilizer or interfacing and apply that to the back of some fabric and one or two of your existing blocks. See if that gives enough strength to go on with. In a book I just acquired, the author recommends Sulky Soft 'n Sheer stabilizer.
Good luck!