In order to qualify as a 1099 (independent) contractor, the couriers would have to carry their own insurance including general liability insurance or a surety bond to cover the potential damage from the loss or damage to the property under carriage. The contracting party would only be able to dictate timely delivery in the contract, but not the means or method by which the delivery is accomplished. Also, the contract would have to be for a fixed period, and payment would have to be based upon the delivery period. Example:
Each delivery could be a separate contract, exactly as with UPS or FedEx, where the shipper effectively engages in separate shipments even though there may be an "open account" containing numerous transactions, which is payable monthly. Or, the contract could be based upon a fixed period of time (month, year, etc.). But, under no circumstances, can any contract contain an "at will
clause. Proof of the contracting party's right to terminate the worker at will almost always transforms the relationship from independent contractor into employee.
At base, the worker must be in business for him/herself as a courier service (incorporated, LLC), and he/she must hold out to the general public to provide courier services for anyone who is willing to pay. You can have no exclusivity over the worker's time.
Please let me know if I can be of further assistance.