The 2013 blog post is probably not what you should be looking at for this...
Try https://developer.roku.com/en-ca/docs/developer-program/roku-pay/implementation/overview.md
For local testing you can turn on "FakeServer" and create some XML files which it'll use for dummy responses: https://developer.roku.com/en-ca/docs/references/brightscript/interfaces/ifchannelstore.md#fakeserve...
You can also make real purchases from a locally-installed channel if the ID matches that of a channel you've set "designate for billing testing" on in the Roku dashboard. If you set up test users there too they won't pay real money, and you can invalidate the purchases for repeatable testing. (I've found in the past that the "designate" thing seems to take up to 24h to actually start working, which is frustrating.. If you're sure it should work, give up and do something else for a day then try again.)
[later] ...editing to add, you will only be able to do local testing of purchasing on a device which is associated with the account that owns the channel you're working on. So, not on your personal device if it's your employer's app and not on your employer's account/device if it's their customer's channel/account - even if you have permission to access their dashboard. I had to discover it for myself years ago, maybe it's documented now.