Can a streamer buy an extension product?

[Dist]

No, you can’t use a payment processor within your extension, as per the guidelines:

5.2 Extensions may not transact or encourage the transacting of monetary exchange in relation to any non-Twitch/Amazon commerce instruments. Additionally, commerce instruments may not be used without being accepted into the Twitch Extension Commerce Beta Program.

If a streamer has contracted you to develop an extension then you can get paid to do so, but all such transactions have to take place outside of Twitch and can not be done within the extension itself.