I think you're right. Just wasn't willing to mess with the motors. But looks like it would be much cheaper and eliminate another electronic board in the chain, reducing chances to further troubles.
I too often find international postage prices being irrational. Sellers often work with a single freight company and aren't willing to check other options even when it's obvious that the price they are being quoted is way exaggerated.
However, I googled for commutation encoders and came up with a much cheaper one from US Digital with technical specs same as the Renco:
And it's priced almost half the Renco, at $161.