Formulas on Numbers to subtract a percentage? and conditional fields on Numbers? (4 part question)

Question # 1 

First I am trying to figure out how to do this formula in numbers so that my “selling fees” column gets calculated automatically.

For example....

The columns I will be using are:

“Selling Price”

“Shipping Price”

“Selling Fees” (this is where I want the value of the formula to be shown)

The Fees I’m trying to calculate are:

10% is the fee taken from depop (so -10%)

And then from that amount 

Minus 3.49% and then minus $0.49 (which are the fees taken from PayPal after the transaction has passed through depop and the have taken their 10% cut.)

So basically this is the math I want to use…

“Selling Price” + “Shipping Price” = X  

and then  X - 10% (Depop fee) = Y  

and then Y - 3.49% (PayPal fee) = Z 

and then Z - $0.49 (2nd PayPal fee) = “Selling Fees”

How do I convert this math into a formula for numbers, to auto calculate the value for Column “selling fees”, using the following 3 columns only: 

“Selling Price”

“Shipping Price”

“Selling Fees” 

As I don't want to have to make additional columns for X, Y, and Z

Question # 2

I will be selling on two sites and the second site (Poshmark) fees work like this…

For sales under $15, they take a flat cut of $2.95 from the order price. For sales above $15, they take a cut of 20% of the order price and you keep 80%.

So how would I do this type of formula using these columns…

“Selling Price” 

“Selling Fees”

So first if the item price is over $15 I would need a formula such as… 

“Selling Price” - 20% = “Selling Fees” 

And if the item price is under $15 I would need a formula such as…

“Selling Price” - $2.95 = “Selling Fees” 

How do I do that on numbers?

Question # 3

Now on-top of those formulas how would I set up a condition? Formula? So that it automatically uses the correct formulas depending on if the item was sold on Depop or Poshmark (this is another column I have added) 

If “sold on” is “depop” use formula A for the value of “selling fees”

OR

if “sold on” is “poshmark” use formula B for the value of “selling fees” 

And lastly

If “Sold On” is “poshmark” 

AND 

“Selling price” is equal to or less than $15 use this formula of “selling price” -$2.95 = “selling fees” 

OR

If “selling price” is greater than $15 to use this formula of “Selling Price” -20% = “selling Fees”

Question # 4

My last question! 

I have a column for the “weight of item” and a column for what I will set the shipping fees at as “shipping price” 

How would I make conditions? Formula? for this? As an example.

Items weighing 50 to 300 g the shipping fee would be $12.99

Items weighing 300 to 500 g the shipping fee would be $14.99

Items weighing over 500 g the shipping fee will be $16.99

How do I set up these conditions so that when I add the weight of an item under the “item weight” column it automatically sets the shipping price in the corresponding “shipping price” column, following the above conditions?

———

I know this is a lot but I’ve been googling for days and I can’t figure out my exact questions anywhere… I’m sorry if that’s all really confusing or maybe this isn’t even something that’s possible to do a Numbers, but it seems like it would be possible. I just don’t really know how to use Numbers yet. So if anybody could help me out I would greatly appreciate it. If I need to clarify anything or you have any other questions, just ask and I’ll be happy to do my best to explain further. 

To anyone that comments or can help, Thank you so so much!