Is there a way to update my quantity field in the payment wizard automatically?

Hi - I have a form that charges per guest. Is there a way to update my "# guests" in the payment portion based on "how many guests will be coming?" chosen earlier in the form?

Unfortunately, that's not possible with conditions. 

However, there's a workaround for that by using a standard radio button field as your product field. Then calculate the total base on the selected number of guests using calculation widget. Guide: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Then pass the total calculation into the payment field. Guide: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

Here's an example with this workaround applied into a cloned version of your form https://www.jotformpro.com/form/63033119143950.

Feel free to check and clone the form back if you like.

Thanks!