Look at it this way. If you'd wanted sick chicks you'd have asked for them. But you paid perfect money for perfect chicks. Sure, things can occasionally go wrong but if low-grade mycoplasma is inherent in the flock and she's buying in eggs which may be from breeding stock that's not been fed correctly or has genetic weaknesses which would make the chicks extra susceptible to such infections, then she should have a mechanism in place for replacing or refunding. Temperature forecast for tonight in the Welsh Marches is just 9 degrees. I have some 5 week old chicks in a coop inside a sheltered Barn which I'm confident will cope but I wouldn't like to risk younger ones.