Sum up customer specific costs from another table

Hey there,

I'm having trouble summing up customer specific costs from another table.

I have a table with loads of columns, the revelent ones are: Order ID, Customer ID and Cost

There can be multiple Order IDs per Customer ID and each Order has a Cost.

I have created another table with the Customer IDs listed. Now I need an array formula that sums up the Costs of all of the Order IDs for each individual Customer ID. 

I know this is possible with grouping Customer ID and aggregations for the sum of the Costs column.

I have to use array formulas. This is my best effort until now:

{=SUM(INDEX(Tabelle1!U2:U15,MATCH(1,IF(A2=Tabelle1!F2:F15,1,0),0)))} 

I know there a few problems.

1. I don't know how to get the IF to return the row number for only the true values

2. I don't know if the INDEX can give back multiple values from different row numbers

I would appreciate help.

Regards Tim