Free for U3 device

I have the Avast AntiVirus for U3. It tells me I have 15 days left, but when I click on purchase, it takes me to a site where there are no purchases for it.
I have a Sandisk 4G U3 USB drive and I need it to be virus protected for when I go to customer’s unprotected computers.
I have written to customer service but get no reply. Could someone help me?