Don’t send shoppers off the checkout page.

That’s the golden rule of e-commerce. However, Tracy DiNunzio, CEO of web-only luxury goods consignment retailer Tradesy, found an exception to the rule.

DiNunzio added online payment service Affirm Inc. to her site in 2014. The service functions as a lender that allows shoppers to break up the cost of a purchase into installments rather than pay the full amount upfront with a credit card. If a shopper chooses the Pay in Installments button on the checkout page, she leaves Tradesy.com and is taken to the Affirm website, applies and finds out within seconds if she is approved for credit financing.

DiNunzio was nervous about shoppers leaving her site, but with Tradesy’s high price points—a used Prada bag sells for more than $1,000—she wanted to offer more flexible payment options that could appeal to different customers. She tested the feature and found that customers who use Affirm spend more than other shoppers. For instance, the average order value for shoppers who used Affirm to buy was $570 last year, more than double…

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