I have a handful of Avada themed websites I have been working on for a few months. Each is a separate mini business I will be launching over the next few months. (Service businesses mostly).
I have an Asian web developer – he is affordable and I like creating a little employment.
* I tell him the content, images, tabs, links, other stuff I want. As well as changes or redesigns. And he does the wordpress stuff. He does a good job for me so far and we get along.
* He would also be in my account to make additional edits and additions.
* **My concern. Now I have to set up payment buttons and product type pages with live links to my Paypal account and Stripe account. Him doing this and or at least having continuing access to my host account and these websites.**
* **So, how do I have him do this or have access to my accounts and these websites, and prevent any hanky panky or theft or mischief that would give him access to the Paypal/Stripe accounts, or customer credit /debit cards, or other things???**
* Maybe some kind of encryption or password particular pages or parts of page? I am more a designer than WorsPress guru

Stripe uses an API key – your dev can’t do anything nefarious with it, nor do you need to give them access to your Stripe account. I don’t know what PP does – haven’t used it for a long time. No CC information is stored in your WP site.
You need to give your developer access to your wordpress site – likely admin access if you’re talking about redesigning it. If they’re just doing content, then Editor role is all they need.
Any code safeguards you make he can override pretty easily.
Best you can do is to have different environments but still if he wants to be malicious he will find a way.