Which service includes the option to purchase up to $5,000 additional insurance?

The service that includes the option to purchase up to $5,000 additional insurance is Priority Mail Express International. This service not only offers expedited international shipping but also provides options for additional insurance coverage. This added insurance is particularly valuable for items of significant value that are being sent overseas, offering peace of mind to the sender regarding potential loss or damage during transit.

Priority Mail and Global Express Guaranteed have their own insurance policies, but they either focus on lower coverage limits or do not provide the same level of additional insurance options for international shipments. The Parcel Return Service is designed specifically for returning items and doesn't offer the same insurance features as Priority Mail Express International. Thus, the correct answer emphasizes the ability to safeguard higher-value items when sending them internationally through this specific service.