Hmm, shapeshift api doesnt provide DOGE_USD . I guess you need to create your own mathematical function that will resolve DOGE to USD.

First you need to get the value of DOGE per 1BTC by getting the value here:
https://shapeshift.io/rate/btc_doge

{
pair: "btc_doge",
rate: "1038281.25000000"
}

Now that you know the DOGE/BTC rate..

Next is to get the rate of USD to BTC by visiting https://shapeshift.io/rate/usdt_btc

{
pair: "usdt_btc",
rate: "0.00040739"
}

Just apply some math, its like converting feet to inches to centimeter