FerryGoGo 
With Eurostar Snap, you can book a ticket up to two weeks in advance. You'll be informed no later than 48 hours before departure which Eurostar train you'll be travelling on. This means Eurostar selects your train on your chosen day or time slot. By doing so, they fill empty seats, and you get a great deal on your ticket.
Keep in mind that Eurostar Snap tickets are non-exchangeable, non-refundable, non-transferable, and non-cancellable. If a friend can no longer join, you won’t be able to transfer the ticket to someone else.
For full details and conditions, check the Eurostar Snap page.
Dover-Calais
Helsingor-Helsingborg
Puttgarden-Rodby