Sendwithus unlocks potential for testing that is largely ignored by the marketing industry.

– Christian Croft, co-founder of uSell

Online marketplace uSell achieves 24.5% increase in open rate and 8.6% increase in revenue-generating offline conversions through Sendwithus transactional email A/B testing.


We love our friends at uSell, a marketplace company that makes it easy for their customers to sell their used gadgets by connecting them with hundreds of professional buyers so that they can get the best offers. They are excited about transforming the way people change up their technology, and they believe in giving their customers the best user experience. By empowering their customers to sell their old things, they’re encouraging environmental responsibility while enabling their customers to use the cash to buy new things. At Sendwithus, we love what they do. And we found their transactional emails quite interesting.

They told us about their ‘retention email campaign’. This drip campaign occurs once a user signs up to sell a device and a prepaid shipping kit is mailed to the customer to complete the transaction. These emails gently encourage customers to send in their devices (free of charge), since an unshipped device is lost revenue.

The Challenge

uSell wanted to focus on their ‘send-in’ rate, the specific metric used to measure how many devices are sent to uSell after customers receive the shipping kit. Since uSell’s revenue depends on customers sending in their devices, they wanted to optimize the call to action in their retention campaign. The goal is to nudge customers into finishing their sale and getting paid. Since the retention drip campaign was the only source of communication with customers after they leave the site, it was critical to ensure the emails were clear. Customers had already agreed to sell a device and  their shipping kit was likely sitting forgotten on the kitchen table. How could they effectively remind the customer to take action?


Improve send-in rate.

The Hypothesis:

A/B test a variety of subject lines to find the message with the best response.

What is Email Subject Line Testing?

Marketers concentrate on email open and click rates as indications of how well an email is performing. At Sendwithus, we encourage customer-focused teams (typically marketers) to look more closely at these metrics as well as to consider using more granular analytics to better understand email performance.

If your open rate is high but click rate is low, it is likely that the content in your email body is not encouraging your customers to take action. Maybe there is too much text and not enough direction for the customer. If your click rate is high but your open rate is low, you could benefit from looking at your subject line. Similarly, if both open and click rates are low, you could definitely improve your subject, body, and call-to-action (CTA) to increase customer engagement.

For transactional email, a high click rate but low open rate might be an indication you should try subject line optimization using A/B and multivariate testing.

The Test

uSell noted that their retention email campaign was critical to their send-in rate and definitely had room for optimization. They decided to try two separate A/B tests that compared the performance of three separate variants.

In their first test, they noted the winning subject line, “Your old device is saying, only 1 week left”, not only had a 24.5% open rate improvement, but those users also had a 7.5% increase in send-in rate.

Their second A/B test concluded with a 14.3%  increase in open rate with the subject line, “We still haven’t received your {{product_name}}”.  Most importantly, that email triggered an 8.6% increase in customer send-in rate.

What We Learned

We were encouraged by uSell’s thoughtful approach in looking beyond open and click rates. Best practices in email marketing focus on the basic testing of frontend touchpoints, like open and click rates.  However, in service-oriented businesses, it is more important to think about action and note that true engagement might need to be measured elsewhere.

We know that transactional email is important because it sees higher open and click rates since these are the emails people want to see (payment receipts, password resets, signups etc). Because transactional email is critical to various engagement points, (in uSell’s case, critical to a customer sending in a device), it is very important to measure the performance of individual emails.

What’s especially encouraging about uSell’s case is their ability to use transactional email testing to increase engagement, while using Sendwithus to test for an offline conversion point.