You can describe GET requests as follows with cURL, which I find a bit smarter :-)
curl -G https://www.shirts.io/api/v1/quote/ \ -d "api_key=APIKEY" \ -d "garment[0][product_id]=3" \ -d "garment[0][color]=White" \ -d "garment[0][sizes][med]=100" \ -d "garment[0][sizes][lrg]=50"
You can describe GET requests as follows with cURL, which I find a bit smarter :-)
In cases where you need to url-encode the parameter, you can switch out -d for --data-urlencode