I think I already know the answer, but is there a way to use Expedited Parcels in Canada to calculate shipping but at the same time cap the amount charged if a Canada Post Flat Rate Box can be used. Thanks

Unfortunately, no. I've suggested that in the past to the shipping team but they indicated they had no plans to implement it. The impression I got was that eBay wants everyone to be using eBay Labels, so while they allow other shipping options to be used they aren't overly keen to encourage it - even if it would enhance sales.

Utilizing Canada Post Flat Rate Boxes | Selling

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Utilizing Canada Post Flat Rate Boxes | Selling | eBay Canada EN