I'll tell you my experience. A friend sent me a scroll and I re-upped on my account. I logged in and since I already had a level-capped hunter and another at almost 80, and since my friend was on a different server, I decided to take the boost to 80 on my bank toon - a hunter. I ran around and cleared stuff out of her bank vault that I wanted to leave on my then-current server, and sent her a couple of things to take with her.
If I recall correctly, you go to your main account page and there is something there that you use to choose the character you want to boost. So, my point is, just don't boost any of your characters. I don't know if it ever expires, but if not then you can keep it for later. I don't remember getting any nag screens reminding me, and I know I waited a few days because I transferred my main character to my friend's server first and started running stuff with him. So I think it's a pretty benign reward that you can ignore if you want.
I hope this helps, and welcome back.
Solicitations malefactors! I am endeavoring to misappropriate the formulary for the preparation of affordable comestibles
Write it in English, not in Pentagonese. It's not a launcher, rifle cartridge, 7.62mm; it's a rifle. Call it a compass, not a direction-finding module. And if someone insists on being called AdcomphibsPac, the Fact File should explain to those not gifted in garblespeak that he's talking about the administrative office of the communication services for amphibious forces in the Pacific.
— Preface to The United States Department of Defense Fact File