Currently Google Apps Script has a computeHmacSha256Signature() function but Amazon's AWIS requires a SHA-1 key for its signature which can be obtained from jsSHA. I've clipped the jsSHA code from here as its too lengthy. You can download it from sourceforge, copy-paste the code from /src/sha1.js. A small change is required though. Change b64pad="" to b64pad="=".