Please elaborate as I was also looking at this.
Newegg has limited promotions to a single code per transaction to prevent "double-dipping" or "promo stacking", so if you bought 2 items that both had codes, you would have to do them in 2 transactions.
They've also prevented the use of promotion codes with items that have bundle items, like free games, or free gift cards in the past. Sometimes you can use their email promos, but general % or $ off promotions like Visa Checkout or MasterPass typically do not work. They will throw an error specifically saying you can't use a promo with that item because they count the game or gift card as a promo.
Rebates are fine though.
Some people in the past have gone as far as trying to talk to Newegg CSR to remove the bundle items so they can apply discount codes. On some of these really hot, new items like GTX 980, it's pretty safe to assume Newegg has attached minor gift cards ($5) to prevent the use of much larger promotions they are running ($20 VCO, $50 New Newegg Business) because they would rather take the $5 hit over the $20/$50 on something they will sell regardless.
Lower end products like 970 do not have these restrictions however, so no game bundle and no $5 gift card mean you can apply that $20 VCO discount to them to get their effective prices down.