I’m glad that you are receiving those requests.
To answer your question, a quick Google search shows this explanation:
“What is the difference between Venmo and PayPal?
While both PayPal and Venmo are digital wallets, the main difference is that PayPal is widely used by small and large scale businesses, whereas Venmo the majority of the time is used for transactions between friends.”
PayPal is easy for you to create a standard (lowest level, no monthly fee) account and link it to your checking account.
Then whenever you need, you create an electronic invoice to email from PayPal to the customer. They receive a secure link to click and enter the credit card info.
“Creating and sending invoices is free. Plus, there are no setup,” — But they still will take a nearly 3% credit card transaction fee.
I like using Square for scheduling appointments and most frequently when creating/sending an electronic invoice. They also take a nearly 3% credit card transaction fee.
Zelle is self explanatory/easy to send from a bank account to another bank account, without a transaction fee. But I’m not sure you can collect a credit card payment with it.
I haven’t used Venmo.
One more thing, I really try hard to never touch a customer’s credit card, handle their card info, and certainly not store their credit card info. When you store their info, you open yourself up to greater risk if there was a security breach that allowed bad actors to access that info. Therefore, whenever possible, send a secure link where the customer submits their credit card info directly to the merchant transaction provider — that reduces your liability risk and is probably the best way to keep you out of trouble.
In summary, either PayPal or Square would likely work for your need to send a custom email invoice to each customer with a secure link to click and pay by credit card.