@Bob - One thing to keep in mind - GSuite has a rolling 500 emails/24 hours limit and it's likely that if you use it to send a blast like this, you'll get enough people hitting spam early on that the rest of your emails will go in people's spam by default.
You'll want to use something like Mailgun or Sendgrid for this.
To @AndrewJDavison_Luhhu's point, say you still wanted to go with G Suite and it wasn't a massive list, you could probably get away with it, especially for a one-off instance. (I swear it's 1,200-2,000 per rolling 24hr w/ 200/hr limiting or something, though I could be wrong - was just looking into this not long ago).
Just be mindful that you'll have to ramp up the email domain by sending out about 10-20 emails a day, and having recipients both open, and preferably respond to the emails that you send out (recommended that you send these emails to contacts that you know and ask them to reply to the email - this trains Gmail's servers that you are a legitimate address).
There are some email ramp up services that exist, but regardless, Google specifically watches out for people using G Suite to send out bulk emails/spam. I wouldn't even try it unless you first did that though as if the recipient is using Gmail, it will almost immediately mark it as spam, by default, as Google doesn't want you using their email servers for that use-case.
You likely would be better off using a SendGrid/MailGun as Andrew mentioned, and still, I'm sure ramping up the email address would help in that instance.
Regarding USAEpay, how would you be integrating it with Zapier in a perfect work (say via webhook)? My understanding is said email is sent out to the recipient that owes money, they click the link to your website, and fill out a payment form on the website that sends the information through USAEpay. I think you'd have to integrate USAEpay directly to the website, sending payment details to Zapier to go through a webhook over to USAEpay could lead to some security concerns, though I could be wrong. Maybe someone else could chime in here?
Even the Stripe/PayPal integration and such within Zapier is for "new customer" or "new payment" and things like that, not actually used for running a payment using Zapier as the connector. That being said, I'm not even sure a native USAEpay integration with Zapier would solve the situation that you are referring to, if your goal is to actually run the payment through their gateway. There are no "Accept Payment" actions on Zapier for any payment gateway integration where you would actually send through credit card details or anything. It's more for general data, or creating customers on the platform, etc.
@alex - You're right: https://support.google.com/a/answer/166852?hl=en (I was reading an outdated blog post it seems)
Everything you say about the perils of mass mailing with GSuite is correct - don't risk it, and use a proper service like Sendgrid - they're hardly expensive.
I'm going to move this question to the 'Ask a Developer' category as, if possible, the solution to this is definitely going to need some webhook steps so I think that's the best place for it :)
@Danvers I didnt know there was a "ask a developer" category. I only see the "developer discussion" category. Is this a new category you are creating or am I missing something?
Ack! No, there's no new category @ChrisP, I meant Developer Discussion, not 'Ask a developer'. Sorry for the confusion there! There's still a link in the 'Ask a the Community' category that will take folks to the post in its new home, so either way I hope you found it!