Upload & Sell: Off
Piggybacking on what others have said, I think your first task is to work out a very specific arrangement with the pro. I remember attending a wedding as a guest where a relative of the bride brought a camera and offered his photos to friends and family at very low cost, significantly undercutting the pro's sales. Talk about bad blood, even the bride was ticked off.
You may want to agree with the pro in advance not only what you will shoot, but where, how and to whom you will make your shots available. If the pro voices discomfort, maybe you'd want to hand him your cards to process and post. After all, you're doing this as a favor. You're presumably not trying to horn in on his business.
The second matter to resolve is how to shoot. Let's say you agree to shoot only guest candids, leaving all the posed setups and key moments -- family portraits, first dance, cake cutting, garter toss, etc. -- to the pro. For that, you could travel light. You'd need only your camera, one flash and a diffuser. If you can shoot one-handed, holding the flash off-camera (cord connected), so much the better. If you have a battery pack (like a Quantum Turbo) your flash will recycle faster. I'd recommend using One-Shot AF and Low Speed Drive (around 5 fps) so you'll get two or three pops from your flash for each shot. This will usually ensure at least one frame with nobody blinking. Sorry if you know all this already. Not trying to bury you in unsolicited advice. Good luck.