How do I activate the shop with margins?

Short answer: The shop type is chosen on a draft: tick Shop with product margins, tick I accept with these terms, then Save. No putting into service is needed to set the margins afterwards.

Steps

On the “Shops” page, open a draft of the shop — a live shop cannot be edited directly (orange button “Modify the shop (create a draft)”).
Scroll below the collections: the two shop types are presented in two boxes, Standard shop first, Shop with product margins second.
Tick the radio button on Shop with product margins. The terms unfold on their own.
Read the seven terms, then tick I accept with these terms.
Click Save.

You'll know it worked when: Unfolding details shows "Terms accepted on …" where the I accept with these terms tick box used to be.

The choice only appears on a draft

On a live shop, the two-type box is read-only: it only recalls the name of the type in force, with the details link to read the terms again. There is no radio button and no tick box until you have opened a draft.

The type is chosen shop by shop: if you have several, turning margins on for one changes nothing for the others.

What activation changes

An E-shop markup menu appears at the bottom of each product's configuration window, with the amounts available for that item.
Every sale of an item carrying a margin credits your money pot with that margin, less the transaction fees.
Items left at None bring in nothing and cost no transaction fees.

Saving is enough: the E-shop markup menu is available as soon as the draft is saved. Putting the draft into service is what publishes your margins to the live shop, once you have set them.

If saving is refused

The message "You must accept the terms of the shop with margins to be able to activate it" means the I accept with these terms box is not ticked. The draft is not saved and the shop type stays unchanged until it is.

Going back to the standard shop

Tick Standard shop, then Save. The date the terms were accepted is kept: if you later switch back to the shop with margins, you are not asked for them again.

Going back to standard resets all your margins

The moment you tick Standard shop, every margin set on the draft's products goes back to None, all at once. They are not put aside: if you then return to the shop with margins, you have to set them again product by product.