Warby Parker has just announced an important milestone — one million pairs of glasses given away as part of its “buy a pair, get a pair” program. The company aims to make affordable glasses, while also helping to provide glasses to those in need. Every month, Warby Parker donates the same number of glasses sold to non-profit organizations such as VisionSpring, a nonprofit that works in developing countries. VisionSpring distributes glasses, and also educates locals on how to conduct eye exams.

Business is booming for the brand thanks to a loyal customer base. While most people purchase a new pair of glasses, on average, every one to two years, some fans are buying multiple pairs of Warby Parker glasses each year. In fact, it was only this past July that the company announced it had reached its half-million mark — its sales figures have been doubling each year. And with 14 million Americans over the age of 12 suffering from some level of visual impairment, the company has a large potential customer base for future sales.

When asked about how the “math” behind Warby Parker’s giving program works, CEO Neil Blumenthal laid out his thoughts in concrete terms. “The same question could be said of why we offer free shipping to our customers, or why we offer lunch to our employees three days a week,” he said. “If it’s important to you as an organization, you find a way to make it work.”

Part of the secret to Warby Parker’s success is their commitment to top-notch customer service. Not only does the company offer free shipping and returns, but interested customers can have five frames shipped to them for trying on before even buying one. “It’s sort of those moments that we find just win people over and generate good will, so that they’re likely to tell their friends about us,” said Blumenthal.

Over at Wired, Andrew White is one of the few to question Warby Parker’s future. Though he is a fan of the brand, White points out that the cost of shipping pairs of glasses free — that won’t even necessarily be purchased — is a potentially costly offering that would be difficult for the company to retract if the budget got tighter down the road. “It’s a fantastic option, but if everyone else abuses it the way I do, the costs are bound to add up,” said White, who tries on multiple pairs at a time through the shipping program.

Overall, though, it seems like Warby Parker’s decision to combine charity with a sustainable business model built on customer service seems to be working, and will likely propel them into even greater company milestones.