@Daniel Hoffman
I had a closer look to your purchase attempts.
All attempts got refused with "high risk" by our acquiring bank gateway. This does not mean that
you are risky, but our acquiring bank made some bad experiences with your BIN (Bank Identifier Number, first 6 digits of your cc number), e.g. they had several chargebacks in the past.
A BIN says a lot about a credit card: brand, issuing bank, credit/debit, country.... You can see this information if you lookup a BIN database like
https://www.bindb.com/bin-database.html (the image captcha can be a pain, but data is accurate)
To cut a long story short: Our acquirer does not like your bank.
Workaround: try another Mastercard or wait until we are able to accept Visa in the US (~10 days)