Expecting the owner of the envelope to notice the missing treasure and return, Ms Williams waited.
However a week later the precious enveloped remains unclaimed.
"Obviously it is important so we have been looking after it, when no one called we decided to go public," she said.
"We are a little upset for the person it was meant for, we just need to get it back to the children of the letter writer, whomever they are."
The yellow A4 envelope lists the first names of the owners, which have been withheld by the Ms Williams so verification can take place.
"If someone does come forward with those names, we can make sure it gets home," she said.
The unopened envelope is relatively light, and is believed to contain a letter no longer than a few pages.
"Nothing as important as this usually they do get claimed," Ms Williams said.
"It is usually just the old sunglasses, the wallet or car keys, but people come back to grab them pretty quickly."
"We have a few visitors come from Brisbane, or they could be locals, we just hope they realise it is missing so we can give the letter back," she said.
This story was originally published on the Gold Coast Bulletin.
** UPDATE **
Nine News is reporting that "mummy" has come forward and the letter has been returned to the owner.
"Great news, we have reunited the letter with its owner!," said Dawn 2 Dusk Café manager Sharna William this morning.
"'Mummy' would like to remain anonymous but is overwhelmed by the support from us all."
Ms Williams said the mother is in "good health" and that the letter was written for sentimental reasons.