These aren't actually being offered via any store, but DRM-free directly from the author in ePub & Mobi (so don't hammer her site, these are 60 mb zips, at least for the ePub version and I'd advise just picking up ePub and converting if you need both, considering the Mobi is probably an auto-conversion anyway).
She's offering these free through May and June for the Hugo voting season because she's up for a John W. Campbell Award this year if you're planning to vote in the Hugos and wants people to be able to sample her work in full before deciding, which is nice of her.
So don't hammer her personal site with unnecessary bandwidth overcharge.
Here are her
ISFDB entry and what
Wikipedia says (editor of the Escape Pod series of sfnal podcasts, among other things).