Sum values based on a range of data

Hoping to get some help on the spreadsheet I am currently working on.

Basically, I just need a formula in my column D and column G.

Want I wanted is the formula to automatically calculate commission values based on the parameters I set in cells B3 to C5.

Such that it calculates in this way in column D:

First : Compute the commission value of the amount in cell C8 that is less than or equal to $350,000 by the rate in cell D3, (simple stated, compute the first $350,000).
Second : Compute the commission value of the amount equal to or greater than $350,001 up to $500,000 (or actual value if lower than $500,000) by rate in cell D4
Third : Compute the commission value of the amount greater than $500,000 by rate in cell D5.

As for column G,

I would want to get the totals of the commission value per code name but without using column D as reference. Which means the formula in column G has to calculate the values on it’s own.

Note that I would need the formulas to reference the upper cells (B3:D5) as that figures might change from time to time.

Really hoping someone could help me out. Thank you thank you so much! 