Most of the merchandise I've been asked to design has been for bands or music-related clothing companies. Some of these clients came to me with ideas and concepts already in mind for their art, and others asked me to wing it and surprise them. Regardless, they've all been satisfied with their designs and hopefully went on to make tons of cash with them!
For all of my merch-related designing, I use a combination of Adobe Photoshop and Adobe Illustrator. All of my art is modified from photography, drawings brought in by clients (I'm a horrible freehand artist), or crafted using custom brushes and cleaned up to be screen printable.
Rates for Merchandise Design range from $10 for a button design to $50 for shirt / hoodie designs. Discounts can be offered if you need multiple designs, but are not guaranteed. Contact me for a more accurate quote!