Flint uses Stripe for payment processing. Strips accepts most credit and debit cards: Visa, MasterCard, American Express, JCB, Discover, and Diner's Club Cards including international cards.
Therefore you can accept international cards for payment, however, you might want to temporarily disable the US-based ZIP code validation. This needs to be done within your Stripe dashboard at https://dashboard.stripe.com/fraud/rules?rule=block_if_wrong_cvc
Set your "Block if ZIP code verification fails' to DISABLED
Once this is done, you can ask you international client to pay their Flint invoice or Sell Online item. Ask them to enter the US ZIP field with 00000 (five zeros). The transaction should go through, but at least will not fail due to ZIP code validation.