This answer might come a little late for you, but who knows – it might still be helpful. Impala offers a universal API that allows you to read from and write to various hotel systems. This includes MICROS (now Oracle Hospitality) Opera, Infor HMS and many others – and it includes an endpoint to add charges to a room bill.
The way Property Management Systems deal with integrations is typically by offering XML-based interfaces that they give access to at a pretty high cost per month (often in the hundreds of dollars per hotel, per month). So, this might be prohibitive if you want to sell an integrated solution to a hotel (and it is sometimes more expensive than what you would charge for the actual product).
Our integrations (full disclosure, I'm the Product Manager for Impala's API team) make use of a mix of methods that allows us to integrate faster at a price that enables you to offer an integrated solution to your hotel customers. Often, these include the legacy APIs, reading out reporting and exports, and writing back using Robotic Process Automation and human-assisted integration.
If you're interested or have any questions about this market, let me know, I'll be happy to answer them.
All the best,
I'm curious, those charges are already easily posted to the guest folio within the existing hotel systems. What's different with O-Valet? Hotels usually have their own garage structure, so they aren't tracking drivers and times the same way as other curbside business destinations. And yes, most of the major systems have APIs and pathways that have been described by others below.