Simple
Combobox with hardcoded list of types and add a PaymentType to your table

More Complex
Create a new Table: PaymentTypeT
Add all your types
Add PaymentTypeID to your OrderT

Welcome Ben!
  I don't think I've seen you here before.  There is a POT LOAD of info here on the site just waiting for you.  Probably one of the best pieces of advice I could give you is to roam around awhile then learn to use the little Search Box up on the Menu Bar.
Then if you can't find what your looking for - post a question for sure.

If you think about it, the only difference between an Invoice and a Receipt is whether or not it is PAID. (basically)

Here are a couple of videos to get you started.  Be sure to watch the extended cuts since you are a Silver Member.
Invoicing  and Change Orders from Quote to Invoice to Receipt
Do a search for Check Register too.  

Hi Ben. That's just one more type of Relational Combo Box.

Just thinking Cash, Credit, Check... good idea for ABCD in the helper table.  What are the thoughts?

Sure. :)