I've found that Petmountain usually has the lowest price when it's on sale. But if that's the only thing you order the shipping might make it higher than at other sites. So you have to check that out & factor it in. If you have other items you need to buy and they have everything you want they often have a coupon code to get free shipping with $49 purchase. Unfortunately the last one just expired a couple days ago. Right now Excel is 6.73 for 500ml bottle with 6.99 shipping - total 13.72. Or even better if you buy 2 bottles shipping is still only 6.99 brings the cost of each 500 ml bottle down to 10.23 shipped price. They also sell it in 250ml bottles if you'd prefer those .. but 2 of those bottles will cost you more than just one 500 ml bottle shipped cost. According to Seachem, Excel doesn't expire.