Keep in mind that a variable speed blower lets you go up to 1.0" of static pressure. That can help a LOT with marginal ductwork.
And yes, it's reasonable enough for companies to charge to do a full Manual J&D. That can be a couple hours of work, which isn't reasonable on a free estimate. You can pay someone to do it and then have a set of calculations to shop around to everyone, or you can accept bids based on rules of thumb that say Manual J&D will be used for final equipment and ductwork selection. While rules of thumb are not a good way to select equipment, they're close enough most of the time that many contractors use them to bid. So long as they come back and do the full measurements and calculations before actually installing the unit, it's just fine if they want to bid on rule of thumb and eat the difference if the calculations end up calling for a different size unit.