Most major blank wholesalers are not interested in your FEDERAL tax id (you do NOT have to have one or be an LLC to be a legitimate business) ... they are interested in your STATE SALES TAX ID. This sets you apart - that you are a business selling retail, rather than a hobbyist looking for a few cheap shirts.
Price vs. volume depends on how much business you are doing. You can get pretty good pricing from most wholesalers if you are consistently purchasing, even if the volume isn't huge. However, to get good shipping options, you need to meet certain minimums. Talk to a sales rep when you set up accounts for the best options.