Seems to me posters are an item which are just as suitable (of not more, due to resellers) for pre-sale ... since:
1.) The band obviously doesn't want to over-invest in excess inventory (even though I've contend online after show sales would offset excess inventory and provide added SEO value in terms of "spreading their web presence' wings").
2.) I can only speak for myself, but I think many others are in the same boat in so far as not wanting to look after a poster while at a show & often forgetting to pick one up at the end (if they're still available).
I would suggest to Furthur management to integrate a pre-order on this website directly, after purchasing a ticket to a show (provided the poster design is complete, maybe also start ahead much earlier on this), and/or co-market via an email campaign to those who opt in after making a ticket purchase to an event.
There, I've said my peace and got it out!