Hello there, @Iceguy. Let me share further insights if it's possible to let a customer access their account transactions to see what their purchases and payments are in QuickBooks Online (QBO).

I agree with @Fiat Lux - ASIA that the portal where customers like you can access account transactions is unavailable in QBO. For the time being, you'll need to be added as a user to your vendor's QBO account with specific roles and permissions, so you can log in and see your transaction history.

Furthermore, you can create and send customer statements to your customers or ask one from your vendor to show a summary of your invoices, payments, credits, and balances. To do this, here's how:

1. Go to Get paid & pay or Sales, then Customers.
2. Select the checkboxes for the customers you want to make statements.
3. In the Batch actions dropdown, select Create statements.
4. In the Statement Type dropdown, select the customer statement type (Balance Forward, Open Item, Transaction Statement).
5. Select the dates for the statements and the date range.
6. Review the customer's email addresses.
7. Select Save, Save and close, Save and send, or Print or Preview.

To know more about the process above, please see this article: Create and send customer statements in QuickBooks Online.

Also, I'm including this article as your reference in accepting electronic customer payments for online invoices and in-person sales in QBO: Take and process payments in QuickBooks Online.

Please don't hesitate to comment below if you have other concerns about managing customer transactions and user permissions in QBO. I'm always ready to help. Take care, and have a great day, @Iceguy.