If you placed an order and you realise that finally you will not be able to make it to the Collection you need to tell it as soon as possible to your Host.
Normally, if you let him know early enough (at the latest a day before the collection), he will be able to contact the Producers to warn them not to bring your order. Your Host will then tick your order as undelivered and you will receive a refund payment.
If you let them know on the same day, this could be problematic as the Producers might already be on their way to the Assembly. In this case, the only way is to find a friendly agreement between you, the Host, the Producers or even another Member. For instance, you could post on the wall of your Assembly that you will not be able to come and pick up your order and ask if maybe another Member living close by could take it for you. There are many ways to find a solution to this kind of issues.