Amazon offers two different ways to use the showAll URL parameter:
- With a package name
- With an ASIN
As reported in other answers, the package name variant does not appear to work. However, the ASIN variant works fine, at least as a Web store URL (haven't tried this yet with the market: prefix from within an app).
For example, if you search amazon.com for the app Funky Hoops: Free Style (chosen at random), its listing page shows that its ASIN is:
B00DSVS4AQ
If you construct a URL as follows:
www.amazon.com/gp/mas/dl/android?asin=B00DSVS4AQ&showAll=1
it will display all apps made by Solus Games, Inc., the maker of the Funky Hoops game.
Amazon's page describing the various URLs is here:
https://developer.amazon.com/appsandservices/community/post/Tx3A1TVL67TB24B/Linking-To-the-Amazon-Appstore-for-Android
However, that page incorrectly states that the package name will work in concert with the showAll argument, whereas apparently only the ASIN works in combination with that parameter.
I'll update this answer once I have tried the showAll parameter as part of a market: locator from within an app.