If practical, you should shop on Tortola. Lil Bits has a very limited selection and prices are about double compared to Tortola. For instance, a 1 pound box of pancake mix is $3.49 at Riteway on Tortola, and the same box is $10.00 at Lil Bits.
The ferry does charge for cargo. $8/box if Road Town Fast Ferry is running, $10/box if Smith's ferry is running. RTFF and Smith's alternate every 2 weeks.