If your request, the advice you received, your final order and the cost is in writing you have everything on your side. If there is nothing in writing you may have issues.
This thread has a common vein running through it to others I have seen. If you are using a reputable tank maker or one of the ones listed on this site, their good reputation will be worth heaps to them. If you are using someone who can do it on the cheap then they are unlikely to care if they have cash in hand. But once again, you will need written documentation as proof of what you requested compared to what you received.