M&S Delivery Charges

ThinkingOfLinking

In my area, £3.50 is cheaper than the bus and train fare to my nearest M&S so I'd be happy to pay the delivery charge, especially as it saves me having to leave the house.

Publicdomain

As others have said it is because they are shipped to other places for alterations. This covers all bespoke items such as tailored and made to measure items such as jeans etc.
However one thing I would add is that on the website it clearly tells you on the banner for local school uniform this is the case and also refers you to the t&cs at the bottom of the page

Local school uniform (products with school logos or badges embroidered) are not included in M&S’ current free delivery offer. Mixed baskets (those containing schoolwear items included in our free delivery offer as well as local school uniform or other non-qualifying products) will show a deduction of the delivery amount from schoolwear items only. For example, a basket containing one schoolwear item and one non-qualifying item would charge half the usual delivery cost of your total basket.

Unwanted items can be returned for exchange or full refund provided they are returned within 35 days of purchase with a valid receipt or parcel summary document. Please return any unwanted items in their original packaging using the Royal Mail pre-paid postage label provided. If items are returned with an expired receipt or parcel summary document or without any receipt, we will offer you a credit receipt or exchange to the value of the last known selling price.

I suppose I would be easy enough to miss actually no it isn't !!