Do you think Amazon is making its own sunblock? I'm sure many of their self-branded products are being manufactured by name brands as well.

This is common with many white label products and not at all unique to Costco. For example, I know which craft brewery white labels one of Trader Joe's brand beers (each type of beer might be made by a different brewery). Same production line, just different bottles.

Amazon doesn't manufacture their own Amazon Basics goods. They pay for it, albeit at a lower price.