1Password does do something to help combat the problem that OP had... they change the background color of any input that they auto-filled to be blue. Blue is a

1Password does do something to help combat the problem that OP had... they change the background color of any input that they auto-filled to be blue. Blue is a weird color for most sites and it should stand out to the user (generally in a field of white) after they auto-fill a form. I have had 1Password fill in the wrong inputs before and only noticed it because of this feature. I noticed an input higher up on the checkout form had changed blue. It caught my attention and allowed me to fix it manually. Now in this situation, it looks like Substack's UI is to blame. The secret form field appears to be heavily modified from its original form state, probably masking the blue background that 1Password attempted to give it. 1Password is in a difficult situation here. If the autofill doesn't work (because the developer of the website they are filling, uses non-standard naming), then users will blame 1Password for "not working", not their browser or the website developer. 1Password therefore has to make it work with as many forms as possible around the web, most of which are built by lazy developers trying to throw up a website during a hackathon, or from a developer that learned to code from a 1 hour youtube video. It's a tall order, from their perspective, to try to make this work with as many forms as possible, while having the fewest (ideally none) false positives. The one thing I can recommend for anyone who uses any password manager, is that before you click submit on a form that you used auto-fill for, you always look over the entire form again. Now in OP's case, I could see how they could have easily missed the extra amount (again, Bad UI substack), but this is general rule that anyone with a password manager should follow. Double check everything before you submit.