I want to buy Monero using Bitcoin with Shapeshift API :

Here are my steps :

I generate a public address on https://moneroaddress.org/ The address is PUBLIC_ MONERO_ADDRESS

I already have Bitcoin purchased using Coinbase and to buy Monero using this Bitcoin address is this correct :

Reading the shapeshift API (https://info.shapeshift.io/) for buying coins the request is :

url:  shapeshift.io/shift
method: POST
data type: JSON
data required:
withdrawal     = the address for resulting coin to be sent to
pair       = what coins are being exchanged in the form [input coin]_[output coin]  ie btc_ltc
returnAddress  = (Optional) address to return deposit to if anything goes wrong with exchange
destTag    = (Optional) Destination tag that you want appended to a Ripple payment to you
rsAddress  = (Optional) For new NXT accounts to be funded, you supply this on NXT payment to you
apiKey     = (Optional) Your affiliate PUBLIC KEY, for volume tracking, affiliate payments, split-shifts, etc...

example data: {"withdrawal":"AAAAAAAAAAAAA", "pair":"btc_ltc", returnAddress:"BBBBBBBBBBB"}

Success Output:
    {
        deposit: [Deposit Address (or memo field if input coin is BTS / BITUSD)],
        depositType: [Deposit Type (input coin symbol)],
        withdrawal: [Withdrawal Address], //-- will match address submitted in post
        withdrawalType: [Withdrawal Type (output coin symbol)],
        public: [NXT RS-Address pubkey (if input coin is NXT)],
        xrpDestTag : [xrpDestTag (if input coin is XRP)],
        apiPubKey: [public API attached to this shift, if one was given]
    } 

So withdrawal is the address of the resulting coin to be sent to , as buying Monero using Bitcoin, this address is PUBLIC_ MONERO_ADDRESS

But where is the Bitcoin address specified as in order to buy Monero I need to specify this address ?

The returnAddress is optional but should not this address also be specified as part of the request ?

As my funds are linked to my Coinbase should this address not be used in order to buy an altcoin, ifnot then how are fee's reconciled ?