Booking.com provides the option to set up multiple rate plans for the same listing.
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For example, set one price for rooms with free cancellation and another for rooms with breakfast included.
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From Hostify you can control only one Booking.com rate, which usually is the Standard one. New rates must be added from Booking.com Extranet portal -> Rates & Availability -> Rate Plans and must be set to be based on/Linked the Standard one which is controlled from Hostify.
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Also, newly added rate plans must be XML active.
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To change a rate plan to XML active, submit a request via your Booking.com account.