There are only a few true sweater options when it comes to postal uniforms. Both type 1 postal employees (city letter carriers, CCAs, motor vehicle support/drivers), and type 2 postal employees (window clerk, & other retail operations) have options when it comes to keeping warm with sweaters.
Type 1 employees have a vertically aligned USPS logo, and sweaters come in either a lighter jersey knit option, or a heavier bulky knit option. Type 2 employees either have a long sleeve cardigan with a jacquard knit USPS logo, or a sleeveless cardigan option with a jacquard knit USPS logo. All of these options are reimbursable and you can use your postal uniform allowance to purchase them.