For some reason I can't do the pantry trial...
I had the sane thought/issue in the beginning if you have never used it before then its likely you have the free trial, there's no link saying activate free trial like by a Amazon prime trial, so you could have easily opted in for the trial without knowing, that being said just place an order and see if the $7.99 Shipping goes away, which im sure will go away ,and if not just send then an email (just sent a few today and got a response within 5 minutes was surprised to get such a timely response).
Also the whole prime pantry method of $40 for shipping and membership is kinda new it used to be percentage to fill your box and if full ship,so I would assume you have never used it,try adding a few items above $40 And look to see if the shipping credit was added.
Screenshot?
Checkout This screenshot, I would have been in my glory should I have received such a box while I was in sleepaway camp, but to top it all it only costed $5.
They asked for packages so here they come..