I am in a related situation.
I verbally accepted an offer last week for a teaching postdoc, after negotiation over the phone with the Provost. (The Provost part was weird. For a postdoc? It's a small-ish R2. Maybe that explains it?). I was told that I would have an emailed offer letter in one or two days. It's been seven days, and nothing yet.
Is it OK to email the Provost and ask if the written offer will arrive soon? I'm also starting to get antsy with all the work that has to get done in such a short period of time.
I would contact the chair of the department or the head of the lab in which the postdoc is housed -- email to establish a written record and then call if there is no quick response.
The OP on this thread and maybe a few others seem to be unclear on what constitutes a letter, contract, etc. If a dean sends a long email detailing teaching duties, compensation, etc., that's an offer in writing, the same as a letter sent via post. Contracts are what two or more parties put their signatures on.
Someone asked if an offer made in writing is ever rescinded. There have been cases, one specifically in California that I remember, where positions were eliminated even
after a contract had been issued/signed.