In the southern part of Frederiksø there's some old canon boat shacks where you can buy "Ruth's Kryddersild" (Ruth's Marinated Herrings). They are produced on the island and is a Danish speciality.
Try them on a slice of rye bread together with a little cold Aquavit (snaps)
After opening the container you can keep them for one year.
What to buy: You can have 3 types of marinate:
2. With curry
3. With tomato
What to pay: I can't remember exactly, but it was a resonable price.
You can have the herrings sent to you - just call the number to order.
What to buy: Here you can buy postcards, stamps, icecream, a little bit of fast food like Danish hotdogs and other souvenirs.