Ask HN: How to solve the cold start problem for two-sided marketplace?

I'm building a P2P crowdshipping marketplace, basically BlaBlaCar but for packages instead of passengers. Travelers going between cities/countries carry items for people who need to send stuff. About to launch the MVP and hitting the classic chicken-and-egg problem. Travelers won't sign up without packages to carry, senders won't post without travelers available. Every marketplace founder says "focus on one side first" but nobody gets specific about how they actually did it, especially when you