HI,
I finally didn''t use any plugin for amazon payment, & did with
Amazon
simple pay..
It offers a good ruby code, to create checkout button.
here is the code given by amazon for creating button. The Price,return 
url can be dynamically changed.
            require ''base64''
            require ''openssl''
            module PayNowWidgetUtils
            def generate_signed_form(access_key, aws_secret_key, 
form_params)
                 form_params[''accessKey''] = access_key
                 str_to_sign = ""
                 form_params.keys.sort.each { |k| str_to_sign += 
"#{k}#{form_params[k]}" }
                 digest = OpenSSL::Digest::Digest.new(''sha1'')
                 hmac = OpenSSL::HMAC.digest(digest, aws_secret_key, 
str_to_sign)
                 form_params[''signature''] =
Base64.encode64(hmac).chomp
                 signed_form =<<-STARTFORM
                 <form 
action="https://authorize.payments-sandbox.amazon.com/pba/paypipeline"
                       method="post">
                 STARTFORM
                 form_params.each do |key, value|
                   next unless key and value
                   signed_form +=<<-"FORMELEM"
                     <input type="hidden" name="#{key}"
value="#{value}" >
            FORMELEM
                 end
                 signed_form +=<<-ENDFORM
                     <input type="image"
                           
src="https://authorize.payments-sandbox.amazon.com/pba/images/amazonPaymentsButton.jpg"
            border="0" >
                 </form>
            ENDFORM
            return signed_form
            end
            end
            include PayNowWidgetUtils
            ACCESS_KEY = ''<your amazon aws access key>''
            SECRET_KEY = ''<your amazon aws secret key>''
            print generate_signed_form(ACCESS_KEY, SECRET_KEY,
                                     ''amount'' =>
''USD 100'',
                                     ''description'' =>
''Test Button'',
                                     ''referenceId'' =>
''txn1102'',
                                     ''returnUrl'' => 
''http://localhost:3000/success'',
                                     ''abandonUrl'' => 
''http://localhost:3000/fail'')
For getting amazon aws account, sign up for its service here.
http://aws.amazon.com/
any help mail me,
Thanks,
http://srikanthjeeva.blogspot.com
http://srikanthj.co.nr
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