I sent an invite to my accountant. They tell me they have to subscribe and pay for the service. Is this correct?

I can help you out with this, @kevin-lodiiron-c.

QuickBooks Online allows your accountant to access your account without payment.  You can invite up to two accountants to access your account. A few things to note:

1. If you invite your accountant and they don't have a QBOA subscription, they'll have a chance to sign up when you send them this invitation. Signing up is free and allows them access to your QBO company at no cost to you.
2. Your accountant will have access to all of the features available in your version of QBO.

You can read more about managing accountants within your QuickBooks Online account by checking out this article: Managing accountant users in QuickBooks Online.

I hope this helps.  If you need additional clarification, please let me know! I'm here to help. Have a great weekend!

@kevin-lodiiron-c 

If you are using QBO US version, your accountant can open a QBO Accountant US account for free. 

If my client has an existing QuickBooks account and I, as an accountant, send them an invitation to access their account, will the pricing for any additional services be added to the cost of their current plan?

@E F 

If your client has an account, they are the one who can send the invitation. You can only send an invitation to create a new blank account.

Let me provide some insights about sending an invitation link to your clients, @E F. 

If your client has already created a QBO account, we can ask them to invite you to become their accountant. Just like what 4Gal mentioned.  

For more details, check out this article: Add clients to QuickBooks Online Accountant. 

Regarding your concern, if there will be additional costs to their current plan if you invite them as clients, so no, there will be no added costs. The current pricing for their plans will remain as is. 

Furthermore, browse this article to learn how to keep track of clients and update your client list: Manage clients in QuickBooks Online Accountant. 

For further queries about handling your clients, please don't hesitate to post here in the Community space. I'll be here to lend a hand.