Promote an additional product to upsell either during or after the checkout process on your website, if possible.
Is it easier to sell to an existing customer or to a new customer?
Existing. Every time. A smart move that many companies make is to try to maximize sales from one individual through one purchase, rather than trying to convince multiple individuals to each make one purchase.
One way this is done is through "upselling". Upselling is a way of offering another product (sometimes at a discount, but not always!) to a customer throughout their shopping/checkout process on the website.
Depending on your website host, you may or may not be able to have a feature like this. However, you can always send a post-purchase email and suggest additional products to go with the purchase they recently made. It's up to you how you choose to utilize this idea, but again, it's much more cost efficient to try to acquire another sale from the same person.