I added more characters to the main text on each listing to 300. The reward text I will find out. I think there should be enough room to say what you need to about booking online. If you are using Res Key you can add an extra field to your reservation form. I took a screen shot from mine so you can see what it would look like. It only takes a few clicks to set it up.
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Why would you add an extra field and not just create a discount code?
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I guess that would work if you are only giving discounts. Its been really fun to see all the different things people are offering. If I was a guests I know some of them I would want to cash in on!
Not everyone is giving discounts.
If you have a discount code that you mention every where then you wont know where the booking came from. If you choose to do it that way, then make sure you have a different code. This way you can keep track of it. If you have it spelled out on your reservation page of where to Enter the Code, people will know what to do with it. But you could try the discount code and see if people know what to do. They might call you and say "I see you have a place to enter discounts, but I don't see where to enter my InnRewards code?" Time will tell!
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