I need to handle online payments and am trying to choose a payment processor. One requirement is to handle refunds and rebates to the customer. These won't always be at the time of sale, and not for the entire amount of the purchase. Is this something all payment processors handle? I don't want to have to do this manually as there may be many rebates, and they may be for relatively small amounts.

I see PayPal has a refund API, but other parts of their site talk about sending a refund within 60 days. Is this something also required by the API?

Amazon FPS also has a refund API that seems a bit more flexible.

The Google Checkout refund has an amout field, but it's unclear to me if you can do a partial refund as the description reads "The refund-order command instructs Google Checkout to refund the buyer for a particular order."

What are some things to look out for when looking for a payment processor that can handle rebates and refunds? Is there always a time limit in issuing these refunds? Is using a merchant account better for this type of process? I was hoping to avoid that due to the increased cost and complexity, but would consider it if it meets all of my requirements.

Update

It appears the refund process is fairly simple and handled by all processors. Is there any additional information on rebates? I would like to avoid a process of sending live checks to customers, but I will have to send rebates in some small amounts that may be a few months after the initial purchase.