This used to be way more common, but these days people are far more hesitant as the industry has come to understand just how critical the garments are to the actual process. They affect the print quality, potential staining, pretreatment volume, wash fastness and more....... Ultimately, a client's poor choice in blank shirts can easily affect a print shop's reputation (I guarantee the client isn't gonna accept "it was your carded cotton shirts that caused such crappy prints" as an excuse when things go wrong).
It's best to let the print shop select and filter the blanks based on their expertise and experience.