How would I force Mac App Store to save my password indefinitely, rather than for 15 minutes?
This is by design. It is meant to ensure that you indeed are verifying the purchase, especially since it involves money.

If you find entering password painful, consider setting up Touch ID (only applicable on MacBooks equipped with Touch ID) to authorize App Store purchases. 

To configure, go to System Preferences → Touch ID and enable for iTunes &amp; App Store. Also, go through this Apple Support document:


Use Touch ID on your Mac


In case your Mac doesn't have Touch ID hardware, consider using a password manager. A password manager lets you create, save and manage strong passwords for app and websites, and enter it for you automatically. This way you get both convenience without compromising on security.

There are many good third party password manager apps for Mac to securely store your password, so you can copy and paste it if for some reason you don't trust the keychain from Apple.

Search the Web for password manager for Mac.