It’s complicated

I’m a manufacturer and wholesaler of a small and relatively inexpensive product. We’ve always done wholesale, and this year we took the plunge and started selling on Amazon FBM. In order to not undercut our wholesale customers and their retail prices, we priced it on the higher end of our normal retail price range. We have now found that one of our wholesale customers is offering our product for about $4 less on the competing website with the blue and yellow logo.

A few things:

I’m concerned this could affect our Amazon listing/price somehow. I’m concerned this will cost us sales. They were sold to this customer so they could sell it on their own website, and not on another marketplace. I don’t like them selling it elsewhere, and especially not at the price they’re selling it for. We don’t have an official MSRP. Could I prevent this if I had one?

WWYD? Talk to them? Let it go? Not sell to them anymore? Any input is welcome!