You have the form setup for tblCustomers and tblBillings backwards. tblCustomers should be on main and tblBillings should be dependent subform because tblCustomers primary key is saved as foreign key in tblBillings.
Why is CustomerID in tblCompanies?
If you want both CustomerID and CompanyID to automatically populate fields in dependent table, that would involve a compound key. I try to avoid them.
I am still not understanding purpose of tblPayments and tblMarket.
Third time asking: Why does tblPayments have multiple credit card fields as well as fields for Cash, Paypal, etc?