I'm pretty sure you're wrong. I don't know anyone who would consign to Heritage if they were taking 17.5% of hammer from the seller. That would be the highest by a large margin across all coin auction houses.
Your best bet is to contact heritage. My sellers fee was 5% - and I might have been able to get it down lower, but they based that off what I had to be sold and what I had bought from them. Beware that at the time I talked with them they put the more common stuff and lower grade on Sunday and Tuesday's auction. You might also be able to negotiate that. But start talking to them. Keep in mind it will take a while to get your money. I think I got mine about a month after the final closing.
Their T&C are confusing, but here's a link. 15% appears in there in several contexts, but I'm still not completely sure that's the right number. I did find a couple of forum posts that mention 15%, so that's why I said "Best I can find...." I suppose you can call them.
The seller's fee is 5%. The buyers fee is 17.5%. Heritage will negotiate based upon volume. I once consigned my coins through a dealer and the rate was tiered based on the sell though rate of the material consigned. With a 100% sell through rate, Heritage offered 108% of hammer. The dealer's fee was 2%. In order to have you material included in a signature auction, you must have a total value exceeding $5K. And even if you consign over $5K, if the individual coins are under $500, they are usually relegated to the internet only session of the sale. If your material is less than $5K, they will place it in a weekly internet sale.
Generally the larger the quantity and the better the quality of the consignment, the lower the seller commission. Although if you have a very specialized collection, which will attract a large group of new bidders, they will negotiate the commission. That said, $5,000 is a small consignment.
I am very familiar with them. I buy from them every week. They don't seem to get as high sell prices as Heritage though. I am seriously considering it though.