"Dispatch" is used for things that are sent to a specific destination or recipient. Examples: send a package from A to B or send instructions or a report from one person to another. "Issue" is more general and is used when no specific destination or recipient is stated, but the object or message being sent out is intended for a large number of people. You issue an order, a magazine, stocks and shares, etc.
The time between the order being placed and it being mailed out is the dispatch time.
There is none. According to the Oxford Dictionary, both spellings are synonymous for all meanings of the word. However, 'dispatch' is more common in American English and 'despatch' seems to be more common in British English. It really comes down to a matter of personal preference.
When the ordered goods leave the company premises, the authority issues 'Confirmation of Dispatch' to the customer,stating the exact date,time,mode of dispatch and probable time it will take to reach customer's hands.
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