Since there is no complete table definition will be not accurate suggestion from us.plus no idea either you have clustered index on tables or not ?
anyways
Order table will have index on InvoiceID and InvoiceNumber
OrderLines will have ix_test on orderLines(InvoiceId, ProductID,status) Include(.....)
and cardinality high for left most column will work better,like sequence InvoiceId, ProductID,status
-------Bhuvnesh----------
I work only to learn Sql Server...though my company pays me for getting their stuff done;-)