We recently rented cars in 2 different weeks in August. Ours wasn't that much different in price when we rented Seattle to Seattle and Seattle to Portland with Alamo. They did charge extra for extra driver though...even a spouse.
You need the contact the rental car company you are dealing with. Each company has their own set of fees.
Try Thrifty car rental company in PDX location
If I was them, I'd be glad to do it for free for you. But I do think there's no additional cost
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