2 sheets, one with data other creates multiple "invoices"

I believe that this proposal is based on a similar concept to that which Pete_UK suggests.
It appears that someone has put together a concatenated list of client names in column S of the Main sheet.
Using that list put data validation in cell B1 on the Invoice sheet.
1. Produce a named range (Lst_Clients) with a Refers To of: =OFFSET(Main!$S$10,1,0,SUMPRODUCT(--(Main!$A$11:$A$61<>"")),1)
2. The Refers To of the data validation list is: =Lst_Clients
Many of the other cells in the invoice could then be populated using formulas similar to: =INDEX(Main!C$11:C$30,MATCH(B1,Main!$S$11:$S$61,0))
A few cells could be populated based on cells that are already populated, as opposed to getting values from the Main sheet.
For example populate the Total Hours Billed using: =SUM(B10:B14)
After that cells A1:G18 are selected and copied (Ctrl + c)
Select cell A19 and paste (Ctrl + v) etc.
Let us know if you have any questions.