Post Boxes

[Bad de Grasse]

Not sure where to post this, but here goes: I'm leaving on a sunday & wanted to post my portable vape back in a jiffy bag just before I leave. I can get the stamps sorted the day I arrive, but just checked out the Dam Post Office site, which shows a photo of a post box http://www.amsterdam.info/tips/post-office/ & I'm wondering if a small package will fit through the slot? Any ideas/suggestions?