String Trimmers

I just purchased my Mower and Power trimmer/Blower and the store is telling me that they cannot get the Bli150 Battery that I paid for. They are saying it was replaced with Bli20 (4.0Ah) which shows less Ah then then Bli150 (4.2 Ah). I cannot find a ton of information on if this is the case. New model or whatever. Is this a cheaper battery? Should they charge me less?

Thanks,

Marty