Click on cells that stretch across columns to have all of them highlighted. Right Click. Merge Cells. Expression is now shared in scope of cells above it.
EDIT:
Okay you are in essence wishing to show a parent expression below it appears in a matrix. I am not certain if you can do this as a matrix on the detail level is going to be the lowest leaf of a tree structure. You can get totals as you are doing but at the lowest level the matrix is on a detail level expanding out to it's fullest. However you can quite easily populate the total by customer above it as the parent will only aggregate that which is in common with what you tell it to group on and no more. Here are two possibilities.
On your 'name' or similar column right click and choose 'Insert Rows'>(Expand to)>'Inside Group Below' then put your expression in for 'Sum(Amount)' or similar. You now with have a totals showing of the grouping of the names before the details level.
You can also expand and collapse the details level at will with a trick that will show the grouping name UNTIL you hit the '+' sign to expand it. Right click your 'date' field in the 'Column Groups' of the designer. Choose 'Visibility' on the left pane. Choose 'Hide' for 'When the Report is Initially run'. Check 'Display can be toggled by this report item' and choose your parent group of whatever the 'name' of products is.
Mimicking your data I did this: