Have a look on the below information about Shipping method and Ship To address from GP help file. If you want more detail about any field in GP, just click the F1 (Helps-->About This window) to view the help file and find the details about fields and buttons.
Ship to Address ID
Enter the address ID for the ship-to address. You can’t modify a ship-to address for an item assigned to a standard or blanket purchase order that has a Delivery shipping method type. You can’t modify the ship-to address if a drop-ship sales order line item is linked to a drop-ship purchase order or drop-ship blanket purchase order.
The default ship-to address for the first line item on a purchase order comes from the Purchasing Vendor Detail Entry window, except for a standard or blanket purchase order with a Delivery shipping method type. The ship-to address for a standard or blanket purchase order with a Delivery shipping method type is from the site ID for the item. The ship-to address for subsequent line items comes from the previous line item entered.
Refer to the following table for the ship-to address for the line item.

Document type
|
Shipping method
|
Inventory registered
|
Ship-to Address
|
Default Tax Schedule
|
Standard or Blanket
|
Delivery
|
Yes
|
Item’s site ID
|
Site Tax Schedule ID
|
Standard or Blanket
|
Delivery
|
No
|
Ship-to address assigned in the Purchasing Vendor Detail Entry window
|
Company Tax Sched
|
Standard or Blanket
|
Pickup
|
Not applicable
|
Purchase address assigned in the Purchasing Vendor Detail Entry window
|
Purch Addr Tax Sched
|
Drop-ship or Drop-ship blanket
|
Delivery
|
Not applicable
|
Ship-to address assigned in the Purchasing Vendor Detail Entry window
|
Tax Schedule ID
|
Drop-ship or Drop-ship blanket
|
Pickup
|
Not applicable
|
Purchase address assigned in the Purchasing Vendor Detail Entry window
|
Tax Schedule ID
|
All document types
|
Blank
|
Not applicable
|
Blank
|
Tax Schedule ID
|
You can’t modify a ship-to address for an item assigned to a standard or blanket purchase order that has a Delivery shipping method type. You can’t modify the ship-to address if a drop-ship sales order line item is linked to a drop-ship purchase order or drop-ship blanket purchase order.
If you change the document type of a standard or blanket purchase order that has a Delivery shipping method type to a drop-ship or drop-ship blanket purchase order, you must delete the shipping method for the item. After you delete the shipping method, you can enter a new shipping method and ship-to address.
An icon will appear on the Ship To Address ID field if the address information has been changed for the current line item. Changing the ship-to address may affect the tax schedule and line item status.
Shipping Method
Enter or select a shipping method for the item. The default shipping method for the first line item comes from the Purchasing Vendor Detail Entry window. The shipping method for subsequent line items comes from the previous line item entered.
Changing the shipping method may affect the ship-to address for the item and line item status.You can’t modify the shipping method if a drop-ship sales order line item is linked to a drop-ship purchase order or drop-ship blanket purchase order.
If the shipping method is used in tax calculations and you change the shipping method so that the shipping type changes, taxes will be recalculated automatically. Use the Company Setup Options window to specify whether the shipping method influences the tax schedules used to calculate taxes.
If the shipping method type is Delivery for a standard or blanket purchase order, the tax schedule assigned to the site is used in tax calculations. If the shipping method type is Pickup for a standard or blanket purchase order, the tax schedule assigned to the vendor’s purchase address is used.
If the shipping method type is Delivery or Pickup for a drop-ship or drop-ship blanket purchase order, the tax schedule you entered in the this window will be used in tax calculations.
Hope this helps!!!