The moment you click on the '+' a Ticket, Workshop or Add-on will be added to your basket. These will be reserved in your basket for 12 minutes so you can fill in your information without having to worry about them being taken by someone else. This way you can take your time filling in important information like allergies. 
It all comes down to who reserves their Tickets, Workshops or Add-ons the fastest once the ticket sale opens. They can become available again if someone else doesn't complete their order in time!

Payment methods: iDeal, Creditcard, Paypal, Klarna and Bancontact.