Can mount afp share via mount_afp but not via Autofs
Please create a folder Share in the User folder and change your auto_master to

/etc/auto_master

#
# Automounter master map
#
+auto_master                        # Use directory service
/Users/User/Share           auto_nas
/net                                -hosts      -nobrowse,hidefromfinder,nosuid
/home                               auto_home   -nobrowse,hidefromfinder
/Network/Servers                    -fstab
/-                                  -static


and

/etc/auto_nas

Shared_Folder -fstype=afp afp://User:Password@ip/Shared_Folder


enter sudo automount -vc in Terminal with the output:

$ sudo automount -vc
automount: /Users/User/Share updated
automount: /net updated
automount: /home updated
automount: no unmounts


Please regard the additional output: automount: /Users/User/Share updated

Both files need a trailing empty line or it won't work and you'll get the following error:
automount[pid]: map /etc/auto_master: line too long (max 4095 chars) or
automount[pid]: map /etc/auto_nas: line too long (max 4095 chars)

(all tested and verified with a 10.9.5 client and 10.7.5 server)