There's a huge quantity of records at Kew with just a small proportion scanned and available digitally - Lisa's experience is that document she wants to see almost inevitably have to be found in the paper archives ... they can be ordered online but it's a two step process which involves finding the size / number of pages, quoting a price, paying, then waiting for it to be digisited. Once someone has paid for a document to be scanned it goes into the digital record.
The main things I've paid for and downloaded are wills and WW1 records such as battalion diaries. These are not expensive, so dropping the fee isn't such a big deal (unless you had loads on for list). TNA provide this list:
First and Second World War records, including medal index cards
Military records, including unit war diaries
Royal and Merchant Navy records, including Royal Marine service records
Wills from the jurisdiction of the Prerogative Court of Canterbury
Migration records, including aliens’ registration cards and naturalisation case papers
20th century Cabinet Papers and Security Service files
Domesday Book
