Hello Romryan,
If I am following your question, you want to make sure every time that you enter/create a Purchase Invoice for an Item, Fixed Asset, or Charge(Item) the General Posting Setup/Purchasing Account is posted too. I will use the following as an example to explain the setup:

In my screenshot above I show that when I enter a Purchase Invoice and the Vendor is Gen. Business Posting Group = DOMESTIC, an enter any Gen. Prod. Posting Group classification, my Purchase Account will = 54100, except GPPG = Raw Mat. If my Ge. Bus. Posting Group = EU then my Purchase Account = 54200, except GPPG = Raw Mat.
As for Fixed Assets, when you select Type = Fixed Assets, on the Purchase Invoice Line, the entry does not pass through the Purchase Account but post to the FA Posting Group GL Account.
As for the Charge(Item), this will follow the same logic as the Item, except that the Item Charge/GPPG value, which in my example = Services, will produce the Purchase Account = 54100.
I hope the above setups explain how the Posted Purchase Invoice transaction will post to the Purchase Account for Item and Charge(Item) via the General Posting Setup (GBPG and GPPG) combination. If my understanding of your question is not correct, maybe you can write back an example.
Thanks,
Steve