Make sure the Custom transaction numbers option in QuickBooks settings is enabled. This way, Plannit will manage the invoice numbering and prevent duplicates in QuickBooks.
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If you merge clients in QuickBooks, make sure to keep the one that contains the Plannit ID. Otherwise, you won’t be able to export invoices from Plannit to QuickBooks, and you will need to reconnect the Plannit client to the corresponding client in QuickBooks.

Never create new customers in QuickBooks, but rather create them in Plannit, since exporting the transaction will create the customer and the corresponding ID. Remember that you can export data from Plannit to QuickBooks, but not vice versa!

Use only the characters permitted by QuickBooks when creating a customer or product name. For example, avoid using apostrophes or accents in your customer or product names, as this will cause export problems.

Here's the list of permitted characters:
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​Alphanumeric: AZ, az, 0-9
​Comma : ,
​Period or period: .
​Question mark: ?
​To symbol : @
​Ampersand: &
​Exclamation mark: !
​Number/sharp sign: #
​Simple quote: '
​Tilde: ~
​Asterisk: * *
​Space: ( )
​Underline : _
​Minus sign / hyphen: -
​Semicolon: ;
​Plus sign: +

For an invoice to be exported to QuickBooks, it must have an invoice date. This simple oversight can result in the invoice not being exported. If the date is replaced by N/A, no export will take place in QuickBooks.