Is a screenshot sufficient to prove Use in Commerce?

Yes, a website screenshot is sufficient to prove Use in Commerce in the US. However, it must meet the following criteria:

• The specimen and the mark must be legible, not cropped-off, etc.
• The mark in the specimen must match the originally filed mark.
• The screenshot must be real, not digitally altered or created.
• The screenshot has to include a picture of sufficient textual description of the applied-for goods/a clear reference to the applied-for services. In other words, from the specimen, it must be clear what you are offering, and this information has to match your trademark application.
• The mark has to be shown associated with the depiction/description of the applied-for goods/services. This means the mark can't just be displayed somewhere on the page, the connection to the applied-for goods/services has to be clear from the specimen. For example, if the mark seems to be labelling the website itself rather than the product, the USPTO might refuse your specimen. This problem is more common for goods rather than services.
• For goods specifically, the website also has to function as a point of sale, it can't be merely promotional. In practice, this means the screenshot has to include means of ordering the product, such as an "Add to cart", "Get", "Download", and similar button.
• For goods, the prices have to be shown in USD to prove availability to US customers.

We usually recommend website screenshots to clients who offer services or digital products. For tangible goods, we recommend a photo of the product, packaging or label bearing the mark instead. Although a website is a suitable specimen, the additional requirements for goods sometimes make it harder for clients to provide a specimen that meets everything.