Create simple key to match spreadsheet of expen to receipts

I have a spreadsheet here with a list of expenditure and a column explaining where in google drive folder each corresponding invoice is to be found. Someone has already been through sheet and checked that every expenditure over £250 has an invoice / receipt.

I now need someone to create a simple key matching the entry in the spreadsheet with the invoice / receipt at a glance. A simple a -z; aa - zz etc will do fine but up to you. I simply want to see the entry labelled in an additional column and for the name of the document in the folder to include the matching identifier as the first character of the name of the file. I think there are 60 all told. Sums under £250 do not need an invoice but where one exists we may as well add it.

Spreadsheet here - invoice folder on request.