If these tags are to go on shirts that are sold exclusively online, what justifies the added expense for a
pull promotion if the buyer is already aware of your online store?
You could take that extra money, time and creativity and focus on the
packing.
You will still be able to give your shirts that added value and make your online store memorable.
Thank you very much for you advice, but

I can't agree with you in my case, and I can explain why:
to print my web adress on the back of tag will cost me 5-10 buck extra, for 5-10 buck (for small run, for big runs its free) I don't believe you that you can make custom package, but I can for this money make my tag to look better.
And you now .. web adress on hang tag it's a free advertising of your company, and I don't understand company that don't do this, you know .. not only you can see this tags, for example if the tag looks good, not cheap people can keep it.
custom package is a great idea you make some good impression. But they cost, and for a start up company is pretty expensive. I thought about screen printing some poly mailers, I think this is most affordable way to custom you package but I'm not sure if I'll have the money for my first t -shirts..