Help with Invoice Lookup Sample attached

[hullstorage]

Hi there,

i have a sample database to hopefully give you an idea what i am trying to get at:

The sample attached is in various access formats:

i am after, when i open the form for it to allocate and invoice number so when i select customerID this will show any records already entered along with the invoice number

or if this is a new records for customerID then this will allocate the next running invoice number

also dont know if the structure is ok

any help would be appreciated

thanks

simon