Two problems here.
Firstly a startup in a tech field needs at least one solid dedicated technician. Otherwise it's a marketing driven funding mine. This can be a contractor, but best to use just one proven one.
Secondly don't set jobs without negotiating costs.If you know the cost beforehand and the job is done within the parameters then you'd have nothing to complain about.
With your latter case, most contractors will have a minimum bill. Normally this isn't 5 minutes, so expect any short job to be billed at whatever they have, it's usually an hour or two or much more depending on the level. For some jobs I charge a day minimum whether I finish it in an hour or not... If you don't know then ask.