How to add a product as a retail upsell?

Do you want to try tempt guests to order a glass of prosecco with their treatment or purchase towel and slipper hire? 
Within every Offering Type (Appointments, Sessions, Area bookings, Packages & Courses) there is a Related Offerings section. Here you can display chosen Retail products to guests making an order for a specific offering, if they are interested in purchasing the item, this will be added to their basket.
Head to Settings > appointment types/ session types/ area booking types/ packages/ courses > edit > Related > retail upsell >add more and toggle on the products you want to appear when someone purchases this offering > save changes. 
When the customer books this offering the retail cross-sale will be there as an option for them to add to their order. The second screenshot shows how the retail upsell will be displayed on the shop front. It will be listed under the 'Any extras?' section and then the customer will be able to add this extra to their order and checkout.