Those are both acceptable. The first sentence might be better substituting "after your last transaction" for "from last time". But an English speaker or reader would understand both of your original sentences.

good, but only one correction: Tenses are not complimenting each other here, Are you sure there isn't some kind of mistake? Other ways to say this: "are you sure there WASN'T some kind of mistake?" "Are you sure there HASN'T been some kind of mistake?" "Are you sure THIS isn't some kind of mistake?" hope this helps!!