If there's a real intention to send that registered letter now that the reminder has been issued, then the verb is present. " = a plan to do something, a "real" condition for a future action. Isn't that just another way of saying that "should" has been omitted?
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If there's a real intention to send that registered letter now that the reminder has been issued, then the verb is present. If the speaker has some doubt that we'll ever get around to sending the letter after all, then the verb is past:
• "It's high time we send him a registered letter." = a plan to do something, a "real" condition for a future action.