Firstly we need to clear something up, this wasn't before the 'advent of writing'. We first hear about establishments that we'd recognise as inns, taverns or hostelries in the 12th and 13th century. It's been suggested that the Merchant class at this time had a Latin literacy rate of about 40% and an English literacy rate of around 50%. It's not clear just how literate that was but it suggests at least a basic knowledge of reading and writing. In any case it wouldn't take a high level of literate education to be able to keep a tally for tables, rooms or customers.

If credit was given, and there are few suggestions that it was a common facility, it would be to well-known individuals of status who were good for the value (usually via their purser) or to customers whose horses were in the stables. It would be more usual for food and drink to be provided for a table with payment up-front, or it may be included in the board (originating from trencher boards used for serving food) and lodgings fee, also paid up front.

​

^(Sources:)

^(Inns, innkeepers and the society of later medieval England, 1350–1600 (Journal of Medieval History Vol 39 Iss 4); Hare, J; 2013)

^(The Merchant Class in Medieval London; Thrupp S; 1948)