I assume you’re talking about the Star + Galaxy packs: Can only be applied ONCE per account. I agree that saving 50% is a great deal but in all actuality you only get 1600 PLEX and 4 Multi char skill you can do with as you please.
Assuming that you are going to sell you characters from your previous accounts, you’d have to add the cost of that to the price of the items you plan to buy. so if you have more than 3 characters you “need” to sell you’d actually save more money by not buying the packs.