From the pk and fk fields you have defined, it appears that you have this set of relationships:
If you base your layout on Visit, you can place a portal to Charges on this layout to list the charges linked to that visit. Charges::fkinventoryID should then be set up as a value list for selecting items from the inventory table you don't list as a table here.
Is really the same structure found in the Invoices starter solution though the table occurrences have different names. The FileMaker 13 invoices starter solution, for example, has:
So you may want to examine the Invoices starter solution for ideas on how you might set this up.
Making great progress! Now, what I'd like to do in this portal is pick the herb brand and then have the ItemName pop-up only display items that match that particular brand...how might I configure that? (See attached portal)
You are describing a conditional value list. Here are my links on the subject:
There's a lot of overlap between the first two links so you can read one and skim the other. The demo file gives you several examples of different conditional value lists so you can examine them to see how they are set up. If you are using FileMaker 12 or later, you can open this demo file from your File menu to get a converted copy of the demo file that you can examine in your version of FileMaker.
The next to last link discusses how to set up a chain of conditional value lists where the value selected in each conditional value list controls the values listed in the next value list.
My most recent contribution is how to use ExecuteSQL to set up conditional value lists that aren't readily possible with the other methods shown here.
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
New!!! Using ExecuteSQL to produce a conditional value list: Using ExecuteSQL to Produce a Conditional Value List
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.