As @KRobert states your customer will have 24x7 access to support. If you are looking to uplift the Advance replacement on the hardware then you need to look at Meraki Now. The Meraki Now service summary is here, https://www.cisco.com/c/dam/en_us/about/doing_business/legal/service_descriptions/docs/meraki-rma-on... and it does mention 8x5x4, but in my experience this isn’t widely available. In addition, if you read the fine print (RMA requests need to reach the local depot by 1pm), it’s barely worth it - you’re better off going with 24x7x4.
If you want to check the available RMA services you can always check in the Cisco Service Finder tool, https://connectthedots.cisco.com/connectdots/serviceWarrantyFinderRequest?fl=sf.
To be honest, if the customer has more than a couple of devices that are the same model they may well be better off self-sparing (obviously dependent on the price of the devices). This often works out as cheap, especially over a few years and can result in a speedier result. Then you just use the leisurely warranty RMA service.
A spare device doesn’t need a license if you don’t add it to a network. Best practice is to have a separate organisation for spare devices, with no licenses, and assign the device to a network in that organisation - the spare organisation will go into license error (which doesn’t matter), but no-one will be able to ‘accidentally’ claim the device (which they can if it just sits in your inventory without being assigned to a network).