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Please know that the option to select the appropriate printing tray when printing checks is currently unavailable in QuickBooks Online. That said, you can contact the support from your printer provider so they can help configure your print settings. They can also provide relevant information or workarounds to address your concerns.
On the other hand, you can refer to these resources for guidance on resolving printing-related issues in QBO: Fix QuickBooks Online printing issues on browsers for Windows.
Keep us updated if you have further questions or clarification when configuring print settings or other concerns in QuickBooks Online. We're here to help.