That's exactly what I did. I submitted a PDF file with all the receipts. The letter I got said the same thing. They're probably banking on the fact that we won't reply and forfeit our claim.
Perhaps they're doing this so that they can keep the money for themselves? I don't know how settlements work so don't quote me on this.
Yeah I am sure more people will be reporting the same in the coming hours/days. The good news is, they are giving us a second chance to submit our invoices to complete the claim. I'm definitely going to go that route and file.