Officers are keen to identify the woman in connection with the incident at the East Sussex Wildlife Rescue and Ambulance Service (WRAS) store in Terminus Road on Monday, October 15.
A number of clothes to the total value of £32.50 were stolen, police said.
The suspect is described as white, aged around 50, 5ft 5ins, with blonde shoulder length hair which she wore in a pony tail with a fringe. She also wore dark framed glasses.
Anyone with any information is asked to report it online or call 101 quoting serial 20 of 18/10.
Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.