According to the Voyagers Club T&C's effective 28 April 2022 https://www.msccruises.co.uk/-/media/uk/documents/42967_02_voyagers_club_terms_and_conditions_eng.pdf, the discount is for "each" 50 $/€ spent and the discount is applied at the point of sale.
• MSC SHOPS DISCOUNT: for each 50 $/€ spent in any onboard shops you will
benefit of 10 $/€ discount. It applies to Classic, Silver, Gold and Diamond card
holders.Discount will be applied at the point of sale. Non-cumulative to other
promotions available on board. Valid only the first day of your cruise (tobacco and
electronics excluded). Not available to members under 18 years of age.