Need help building a commission sheet

i need a formula that will calculate a tiered commission payout for my company. Ex: the total number of units sold will go in A1, then in B2 will multiply units 1-5 by 15, b3 will multiply units 6-15 by 25, and so on. they want the total for all tiers to be shown in A7 also when tiers 2-6 are reached a bonus needs to appear in C2 -C6 and this total is added in A7 too. Is this even possible? I have racked my brain and searched online and have had no luck.

A1 25

B2 75 C2 150

B3 250

A7 475

so i got the page just about done with some trial and error. my issue now is that i need to have the bonus and activations, etc columns total up, the bonus columns all have if commands and just placing a sum command below the column is giving a "value" error. any ideas? sorry for the multiple questions, total noob here that had this job thrown in his lap. appreciate any help!