A major factor in the ethics involved depends on what the builder expected when he sent the prints, and what the OP was thinking when he requested them. From the original post, they were sent voluntarily, so there's no theft of tangible property or services. Copyright law doesn't apply because the OP isn't commercializing the print. Plagerism doesn't apply because the OP isn't taking credit for work not his own.

The US factory did send out the print in hopes, short of expectation (from the tone of the original post), of an order and had the OP simply not purchased the frame for any reason, no one would be speaking of an ethics violation of any kind. Samples and/or quotes and specs are sent out thousands of times every day in hopes of getting business, but there's no legal, or moral obligation to buy.

So any talk of loss, theft, or copyright infringement is irrelevant up to that point, and what the original poster does with the prints afterward won't cause further loss to the company. Certainly he can't claim it as his own, and he can't reproduce it and sell the copies, but he is within his rights, having acquired them fairly, to use them to his advantage, and have a frame built for his own use.

It would be different if he had made a promise to buy a frame, or put them to the effort of drawing up plans specifically for him, or even if he requested them knowing in advance that he had no intention to buy, and instead use them to copy. But that isn't what happened (according to the OP).The OP's intent is what determines the ethics of the situation, more than issues of copyright.