It varies depending on the album. Recent purchases I've made have been encoded using LAME 3.97 with its V0 setting (~245 kbps VBR), this seems to be the default for MP3s encoded by Amazon. One self-published album I grabbed that was MP3 only was 320 kbps CBR. The MP3's I've downloaded via the site and via the downloader are bit-for-bit identical.
It's a pitty Amazon isn't more forthcoming on what the encoding is before you buy it, but I'd imagine whatever album you grabbed was simply provided to them as a 128 kbps file from the source.