In this scenario that you have mentioned, there are two things you need: 1 - A custom field to annotate that an administrator has decided that a finance charge should be added or not? This can be a simple boolean field (e.g True/Fale) 2 - A conditional action. You mentioned that you are using an IA app, that might mean that you will have to upgrade to our base plan to facilitate recipe customization.
So what can be done will be to insert a conditional action to check if this particular custom field is checked, and subsequently include an action "add line item" of order item type equal to finance charge to the order (I have included a screenshot for your reference below). If you need the price to be a percentage of the original price of Product A for example, you can simply switch on formula mode to take that into account. Hope this answers
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Sara Khoudary
Infusionsoft IA App Finance Charge when using Infusionsoft Payment Plans
We use Payment Plans in Infusionsoft and have the option to implement Finance Charges.
The current Integration App does not support moving finance charges from Infusionsoft to QuickBooks/Xero.
We would like to request that support is added for finance charges when Payment Plans are used in Infusionsoft. (http://help.infusionsoft.com/userguides/sell-online/billing-setup-and-automation/set-up-a-payment-plan).
With payment plans, the administrator can decide if a finance charge needs to be added to the invoice or not. If a finance charge is added, when a person purchase a product from their website, the finance charge is automatically added to the Infusionsoft Order. The product is that the name of the product for this finance charge is the same as that of the product to which the finance charge is applied to.
For example, if someone buys Product A at $500 and the Finance charge for Product A is $50, Infusionsoft Order should two items for Product A. The only difference is the line item type. One is Product Type and the other is Finance Charge Type.
Here are a list of Product Types in Infusionsoft.