Can I give a free dessert when the cart goes over a hundred?

Yes. Open Marketing → Offers, press Add rule and choose the type Free or discounted item over a cart amount. Set the amount to 100, pick the dessert, set its price to zero or to a discounted price, and switch the rule Active. The offer appears in the cart as the customer crosses the line, and it is checked again on our side at checkout. Set how many times it may apply per order so a 300 cart does not earn three desserts. The Promotions & marketing tab in the docs lists all ten offer types.