Can double vouch; as long as you place your order before your trial expires, you'll get free shipping. Billing is done as soon as you place your order - they can't charge you extra after. They could cancel if there were a price mistake but amazon's never done that to me.
Should add Prime only costs me $40 a year as a college student - you just need a valid .edu address; though I dunno about renewing next year with my state charging tax. I can prob make due with the free super saver shipping if I need something from them