Try it all on at home; put the rest in your bag with prepaid shipping label and pop in the mail. (frockbox.ca)
From there, return your unwanted items for free using the resealable box and prepaid shipping label. (businessinsider.com)
For sellers: Order a free thredUP Clean Out Kit (basically a big bag you fill up with things you're no longer using) and mail it to thredUP using the prepaid shipping label. (babycenter.com)