How much should you tip at a bed and breakfast?

I always tip appropriately (at least 20% at restaurants, etc.).

According to this tipping etiquette guide, most bed and breakfasts in the US are family owned and do not request a tip or even specifically request that you do not tip. Ours, however, does request a tip. There is an envelope in the room that reads, “Thank you for your generous gratuity.”

According to that tipping etiquette guide, if the bed and breakfast does request a tip, they suggest following the same guidelines as tipping room service at a hotel. Under the hotel section, they say that you should tip 15–20% of the room charge.

We stayed here for one night, and our room was around $180. Does this mean we’re supposed to leave $36 in the envelope?! That seems really crazy… I don’t want to undertip and be viewed negatively and not welcome back here again.

We’re about to leave here in an hour or so, so prompt advice would be appreciated. Thanks!