Cannot purchase supporter pack - "Invalid Argument" in Shipping Cost field

I've been trying to buy the $210 upgrade supporter pack for like the last week and every day I get this error after filling in all fields properly (including the full zip code). Sometimes the error is "Invalid Argument" and sometimes it's "An unexpected error has occurred", but I can never complete the purchase.

Please let me know what I need to do to complete this purchase. There's a whole thread of people on the Steam forums complaining about this, it's astonishing that this magnitude of bug has lasted this long, given all the lost revenue.

I've also sent two emails to support and have never received any acknowledgment or response. I understand you guys are probably pretty busy right now, but I would think this is a high priority lol.

Thanks for looking into this.

I've reported this for over a week now and also emailed support, but no answer. Hard to believe, but you can't sell me a supporter pack? Help!