Yes there will be a fedex in Mexico and they will deliver across the border. If you are mailing drugs forget about it.. guaranteed to get caught. First of all no warrant is needed to open up UPS/Fedex boxes..and secondly packages from Mexico to USA are scrutinized. A dog will pick up the smell easily.
As to your qeustion 'how do i sue fedex while in mexico'...it wont be much dfferent then using it here lol. Go to a fedex store, mail a package. No magic involved.