Hello,

Keeping all your ducks in a row is what QuickBooks Online is designed for. It's great that you'd like to double-check and make sure your attachments are linked to your transactions. 

You can upload an attachment in QuickBooks Online by doing so within the transaction itself. You can also add attachments in the customer information by clicking Edit > Attachments from the customer list. Similarly, uploading an attachment can also be done for suppliers. Uploading an attachment in QuickBooks Online will automatically store that attachment, or link it, to whatever it's been added to. This means that all your attachments will be stored where they were uploaded for easy bookkeeping. 

For more information regarding attachments in QuickBooks Online, take a look at this helpful community article: Attachments in QuickBooks Online. 

Let me know if you have any other questions. 

Cheers!