It will be quite annoying for users testing the service with $5 to run out of credits and being stopped by an empty Kagi search during the decision period. It will be much more comfortable to have Kagi using Google (or any user preferred search engine) as fallback without needing to click any extra button.

My proposal: Show the special page informing about the lack of credits every day (or a user preferred period of time?) in their first search and create a checkbox (that will remember user's choice) to "remember me again tomorrow and redirect to Google for now". After that, the user will be able to search normally for the said period of time and everything will be redirected to Google without any user interaction. Maybe the interface could be worked together with the Offer shortcut to search on other search engine in a new tab idea.

Related idea: https://kagifeedback.org/d/28-automatically-redirect-search-query-to-other-search-engine-when-user-not-logged-in-or-the-asn-is-blocked