Yes it does look as if your PayPal 'FORM' code has been messed up!
Some of the <form action=..." statements are missing in some of the forms, but the rest of the PayPal code is left.
Where I've been playing with my Instant Site it does seem the HTML editor is rather hard work! I've mentioned in one of my other posts about the way the editor 'formats' things, which makes it's use rather tricky.
If you go back in to edit your HTML you can wade through the 'compressed' code to find all your PayPal entries and try repairing the stripped out bits. It may be easier to copy/paste everything into a normal text editor like notepad (or Notepad++) correct things then paste it all back - but it's not ideal.
NB. It doesn't matter what file format your buttons are in as long as the code is correct.
NB2. One good effect of the Instant Site with PayPal is that your header logo within PayPal is hosted on Freeola's secure site (https) which means users don't get a security warning on PayPal :¬) Normally you have to find a specialist SSL host to get around this.
I believe Eccles is currently working on some Instant Site updates so the 'editor' may improve for PayPal and JavaScript etc. in the future.
I've also been working on PayPal today - apart from the frustrations you've seen here, I've nearly had a nose bleed going round and round in circles trying to get to the bit I want on the PayPal sites!
Are you aware of the PayPal developer 'Sandbox'?
This lets you try out your shopping cart etc. in a test environment. Unfortunately this has been running slow the last few weeks (or more) but it's worth persevering with:
https://developer.paypal.com/
Good luck...
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