You can tell them. The very best ones will refuse if they feel the time is not sufficient (this may depend to some extent on how unusual a shape you are). Most will simply do it to your timescale, but you're left with no choice but to accept what they have done when you go to pick up the finished article. Personally, having been left with a useless suit by this route I really don't think the savings made are worth the risk. But YMMV.