Arriving this year for our annual stay we passed, while riding the train (Berner Oberland Bahn), a new parking area at Matten bei Interlaken. For CHF 5.00 per day you can park all day, 5:00-23:00 https://www.jungfrau.ch/en-gb/arrival-at-station-car-parks/parking/park-ride-matten-bei-interlaken/. There are charging stations for EVs.
By contrast, the big garage at Lauterbrunnen is CHF 18.00 for the first day, although that is 24 hours. Also, at Wilderswil there is 24 hour parking for CHF 5.00 per day for unlimited days https://www.jungfrau.ch/en-gb/arrival-at-station-car-parks/parking/park-ride-wilderswil/.
While you still have to buy a train ticket, which is CHF 7.20 without a half fare card and CHF 3.60 with the HFC. If one has a Berner Oberland Regional Pass or Jungfrau Pass, that train ride is included.
No one has been using the Matten b.I. Park-and-ride in the number of times we’ve passed. Perhaps WengeK or Wanderweg could weigh in and tell us if it is relieving congestion in Lauterbrunnen.