David Ellis Dickerson used to create cards for Hallmark, but his tendency toward straight-up, tell-it-like-it-is sentiments for specific and unusual situations did not jibe well with the culture there. (He wrote a funny memoir about it.) He gets to make the cards he wants now, which he presents on his YouTube series "Greeting Card Emergency." Need the perfect card for Darwin Day? A card for your friend who just did National Novel Writing Month? For your ex-fiancée's wedding? Greeting Card Emergency can help you out.
Here Dickerson offers some Mother's Day card solutions for when "you and your mom have had a rough past, and you've sort of made your peace with it now." They are honest but sweet, and could be useful as peace offerings in any difficult relationship. Also, you can make them yourself! Just like you did when you were 5.
Also for Mother's Day, see Dickerson's cards for two-mom households here.