The voucher can only be used online at Skystore.com so any purchases would have to be made here as well so if the film you purchased was a Buy and Keep you can then send it to your box.
So I would get in touch with Sky Store support regarding this to see what they say, they have a live chat you can use that is available from 11am-7.30pm Monday to Saturday and can be found here:
https://www.skystore.com/help/contact-us
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