where your options are so limited, I dont think I would use a recordset. I would just enter the 2 sizes and the two grinds manually as static items in the list.
For the price, there are a couple of way to go. Probably the simplest is to create a new column in your eCaret object that you can send the large bag price to. you could call it "lgPrice" in the add to cart button, pass the 1/4 pund bag price to the Price column and the amount extra to charge for the 3/4 pound bag to the lgPrice column.
so if a 1/4 pund bag is $4 and the 3/4 pund bag is $9, you will pass 4 to the price column and 5 to the lgPrice column
Next, on the eCart object tag, create a calculation that will be used to determine the price to apply the product, call it realPrice. If the size that is passed for the 3/4 pound bag is "3/4 of a lb" the calculation will look like:
(([size] = "3/4 of a lb")?[Price] + [lgPrice]:[Price])
you then have to modify the total Price calculation:
[Price] * [Quantity]
(([size] = "3/4 of a lb")?[Price] + [lgPrice]:[Price]) * [Quantity]
You will want to change the cart display to use the realPrice column in place of the price column, and change the checkout code top pass the realPrice to the gateway in place of the price.