Yep, they sure can!

You need to turn on this feature by logging into Crik-IT as an in-house user with Crik-IT administrator permissions.  Then, go to the Tools tab, and click on the Company Configuration button.  In the Features Settings section, you will notice a checkbox labeled "Show QuickBooks Online payment link on invoices tab".  When this is checked, another checkbox appears.

If the second checkbox is checked, Crik-IT will only show the payment link for invoices that have been emailed from QuickBooks.  Some companies may create invoices before the product has actually been shipped out, then email the invoice once the product ships.  In this case, they do not want to provide a way for the customer to pay for the invoice before the product has been shipped and the invoice has been emailed.

Once you have enabled the QuickBooks Online payment link, customer users will see a "Pay" link on the invoices tab in their Crik-IT portal, like so:

The Pay link will only show for invoices that are marked in QuickBooks as allowing online payment, either credit/debit card payment or ACH payment.  The example below shows an invoice that can be paid by credit or debit card.

Note that, if you have checked the checkbox saying "Only show payment link for invoices that have already been emailed from QuickBooks Online", the Pay link will only show if that invoice has already been emailed from QuickBooks.  If an invoice has been emailed from QuickBooks, the invoice in QuickBooks will have a notation similar to this:

The Pay link only shows for Customer users, not in-house or rep users.

When the customer clicks the Pay link, a new tab will open up with the Intuit payment window for that invoice, like this: