You don't send the mats to them, you find them on the fleet/planet and trade the item for the materials. I've heard of people saying they lost their mats and didn't get the item when they mailed it to the person that was crafting. So always make sure the person has the item on hand before trading.
or just actually look up who is a trustworthy trader and then you don't have to bother with f2f trading. Believe it or not, most people out there spamming what they craft will not screw you over and most likely ruin their business forever just to keep your mats worth a million credits.