USPS First Class Mail takes 1-3 days to arrive however, there are variables that can add 1-3 days to delivery!
It all depends on the distance from the sender sorting facility to a recipient sorting facility and from there, the recipient sorting facility to your address.
Sorting facilities are typically in bigger cities, and are like a distribution hub. So, from the major distribution center to a smaller city this can add a day or 2 on the time in transit.
Other variables include weekends, holidays, and the overall volume of packages in the USPS mail stream. Busier times like holidays, there is more volume (packages) that USPS is handling, and naturally, things will take a bit longer depending on the efficiency and resources of the USPS.
If there are any issues beyond a 7-day shipping interval reach out to your curator!