Also, Australia Post doesn't have any APIs to automate shipping. Girls had to physically go to the post office to ship items, and there wasn't tracking either. This meant a lot of customer service headaches.
Consider one more roll of the dice, this time in the UK. We speak English and we have a postal delivery network.
Rather than chase the $$$ consider taking your idea to Oxfam - a British charity known for selling donated clothes. Your app will enable them to introduce a younger audience to the work they do. They also have people to pick up the phone, a marketing budget and that network of stores.
From a customer standpoint they really might be pleased to pay a premium to Oxfam if their clothes are going to a good home. It is a different 'doing good' proposition. Things that don't sell might as well go to the Oxfam shop as a donation - the app takes people half way there.
If not Oxfam then there are plenty of other charities that might be interested. You might not make money for the VCs of the valley but you will be able to make the thing work, perhaps to feed a few of those in The Global South as a spin-off benefit.
As far as acquiring new users, the idea of an 'Oxfam app' definitely has intrigue, people would do marketing for you by word of mouth. Plus they would 'excuse' themselves to get their next fashion fix.
Just to clarify, I think people should still get some money back, but with 10-25% or so going to Oxfam. Sure there would be postage to pay that might mean people are making pennies on outfits that cost pounds, but this would be psychologically okay if money is going to Oxfam.
You could also pimp the app to show the user/customer how many mouths they have fed through their trades.
This charitable outcome might also work well for those that have put money in your venture. To them they might not have earned $$$ but they will have done something for the greater good.
The UK market is small compared to the US but the geography is a lot tighter. Crack the UK with a partner such as Oxfam and then the other markets will follow, maybe with different partners. I should also say that charities do pay a living wage :-)
Regardless of what you do next, well done for giving it a go and I look forward to hearing about what you do next.
I happen to know someone who worked at the direction of Oxfam in Spain couple of years ago, so let me know if you are interested - it would be a super long shot though.
OTOH, the E3 US Visa is quite simple - it's generally considered to be the easiest US skilled worker visa to get.
Wine is big business in Australia, but where wineries in California seem to have < $10 shipping nation-wide, an Australian winery will cop $30+ to post a dozen to an interstate capital.
(I'm an Australian with one physical product which would be easier to sell if the shipping weren't so expensive.)