Fix Latvian postcode format when creating a shipment

[Saggre]

Should fix the following error response from Smart Send API when trying to create a shipment to a Latvian pick-up point:

{
	"links": {
		"about": "https://app.smartsend.io/docs/errors/ValidationException",
		"status": "https://app.smartsend.io/status"
	},
	"id": null,
	"code": "ValidationException",
	"message": "The given data was invalid.",
	"errors": {
		"agent.postal_code": [
			"The agent.postal code must be a valid postal code in LV (examples LV-1073)."
		]
	}
}

Although the real solution is for the API endpoint called by findClosestAgentByAddress to return valid postal codes.
Let me know if you need more info about this issue.