Downloaded the postal voucher? All you have to do now is safely wrap your item to avoid any damage during shipping.
You can only send one item per parcel. If not, and the parcel were to be lost during transportation, Vestiaire Collective wouldn't be able to compensate for it. Also, dont forget to include all the extra parts in your parcel, as listed in your ad (bill, dustbag, box etc.).
Tip: add these parts in the item itself or in the box that goes with it. For instance, place the authenticity card in the bag rather than in an envelope or in the parcel.
By the way, make sure you pick dequate wrapping for your item to avoid any damage that may occur during shipping. For instance, if you want to send glasses, it's safer to send them in a cardboard box rather than in an envelope.