You could increase order sizes by carefully choosing items to up-sell.

When choosing items to up-sell, they must be:

• Complimentary to the product - For example, if your customer orders a pair of pants - up-sell belts. Don't try to up-sell a dress. Or, if they purchase a mobile phone - up-sell phone protectors.
• 60% cheaper than the product they've ordered. You don't want to heap on a purchase that they can't afford. You only want to enhance their existing purchase.

However, don't distract the customer from the primary goal by up-selling in checkout.