Urgent- Forgot Priority service payment

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Hi all,
Sorry for the urgent information required.
I submitted my application online for husband visa yesterday and paid ihs and application fee but I forgot to pay priority service fee. My appointment is on the upcoming monday, 2 days later.
My wife will post the documents in the UK in the next days.

Is it ok to pay priority service fee after appointment but before sending the documents to sheffield ?

If I pay for priority service before my appointment, will my wife need copy of receipt or anything else?

Can I change my standart visa to priority visa?

Thanks