I have been racking my mind on this for to long and need help!

I sell retail items and have setup a spreadsheet to calculate many things
for me such as postage for a given item.

I have made a cell into a drop down menu that contains all the possible
shipping weights of an item. For example L10 to L200 for Large letter
shipping costs 10grams to 200grams. P10 to P1200 for packets shipping cost 10
grams to 1200grams. (They go up in increments of 10 by the way for a total of
140 possible weights)

What i need to happen is when say L10 (large letter 10grams) is selected
from the drop down menu for it to auto fill in the shipping costs for 1st
class and other shipping services in different cells. I have another sheet
with the shipping cost per weight information to link when needed. I was
hoping to use the shipping costs in the other sheet so if shipping costs were
to change (they seem to every 7 months) i can simply change that sheet to
correct the items shipping cost.

Hope that makes some sence and thanks for your time and help everyone.

Jonny