Since there are 16 oz in one pound (lb) and "Susan's shelf can hold 2 lbs" then you can either convert Susan's shelf into oz, or the doll's weight into lbs. I would suggest converting Susan's shelf into oz because then you are not using fractions.
To convert Susan's shelf into oz- you know that 16 oz are in one pound. Since Susan's shelf is 2 pounds you can take 16 oz and multiply it by 2 giving you 32 oz.
Since one doll weighs 8 oz you take the 32 oz (weight of Susan's shelf) and divide it by the weight of a doll, which is 8 oz.