Their phrase "3 to 5 working days" is based on Monday to Friday only. It's their GetOut card as they work 7 days a week; couriers and RM deliver on Saturdays. And some people work Sundays nowadays. Ambiguous, I think, when it suits them.
I ordered 2 items on Friday within 30 mins of each other. The second one showed 'shipped' within hours and the first still showed 'In process'. Now both items are on the same driver's delivery van????
For my own sanity I take the delivery to mean 3-5 days from despatch. I try not to check when things are in the overall process. I wait for the email notification and count the days from there. I know that may not be the correct interpretation of the delivery but it means I don't get wound up about delivery. Just my way of coping with the world!