Dragging a formula without changing certain pieces.

I have a fixed value in cells C2, C3, C4, C5 containing ebay/Paypal fee structures.

I have a price in cell D9, D10, D11, etc - which does change depending on the thing for sale.

In E9, I have the formula =SUM(C4+(D9*C5)) - adds the value C4 to D9xC5.

I need to greatly extend the size of the spreadsheet - it is a way of figuring out transaction fees.

Unfortunately if I drag E9 to E10, the formula would change to =SUM(C5+(D9*C6)).

I need to keep the C4/C5 references the same, but change the middle D9 reference. I have tried copying and pasting the formula but that didn’t work. Is there a way to do this or am I doomed to manually edit 400 lines?

Thanks in advance.

Solved - needed to be C$4.