You don’t have to disclose anything if you are just a fan. If you happen to love Toyota and write about them, you don’t need to make a special disclosure that you own a Toyota. However, if Toyota gives you a car to use for two weeks, you must disclose this. It is Toyota’s responsibility to tell you that you need to disclose.
Transparency is key – if you are being paid, you must say that you are being paid. If you are receiving products or services, you must disclose such.
There is no difference in disclosure for product vs. services vs. coupons or discounts. You must disclose all of them.
You must disclose somewhere in the post itself. It is not enough to post a blanket disclosure in the "About Us" section or sidebar of your site. The same applies to tweets. You must disclose in each tweet.There are popular hashtags for disclosure, which include #paid #ad #spon and #sample.
You can also go directly to the source - see the FTC and WOMMA guidance documents on disclosure: