Butterfahrten?
Butter trips...trips whose primary goal is to buy goods, either because it can be done duty-free, because price differences make it cheaper or because of local specialities that can't be bought at home. Normally one of the first two reasons.
Yes, "Butterfahrten" (literal translation: butter trips), not to be confused with "Kaffeefahrten". A "Butterfahrt" is a trip onboard a boat just beyond the territorial waters, so one can buy items like liquors etc. tax-free. Long, long ago (probably at the time of the Kaiserreich, or German Empire), butter was also a highly taxed item, that's what they got the name from.
A "Kaffeefahrt" on the other hand is bus tour with a sales show (usually overpriced blankets, pots and pans). In German a Kaffeefahrt is usually synonymous with "rip-off".