So far I haven't needed them adjusted, so I haven't faced up to this. It's definitely a tricky question, since that's usually a service that shops provide for free along with selling glasses, or being likely to sell you more in the future. Maybe they'll appreciate the opportunity to tell you how crappy the glasses are and that you shouldn't go online next time; maybe offering to pay for the service would make sense - car mechanics don't usually have an interest in selling me cars, after all.