Is there a guarantee the bug stays fixed?

Yes. Every fix ships with a written root-cause note and a regression check, and the Bug-Fix Sprint includes 7 days of coverage, if the same bug resurfaces in that window, we fix it again at no charge. If the symptom you saw turns out to be several separate bugs, we tell you upfront and re-quote rather than billing silently. For stores that keep breaking, the retainer tier puts a developer on standby so the next fatal is handled before customers notice.