In Excel pivot tables,
Put the envelope descriptions in "Drop column fields" and locations in "Drop row fields" (or vice versa). Put Quantity in the Data items.
Right-click in the middle of the table and select Field settings from the menu. You'll see that you can summarize the data by count, sum, average, etc.
You can also right-click on the location and description labels and choose which descriptions or which locations you want to see. You could put the Inventory Date as a page field and decide to see the stock of envelopes at all locations that did an inventory check during the last month (for example).
In Access, I would suggest you make three tables:
Locations (at least 2 fields:LocationID (primary key) and Location names, feel free to add state, region, etc.)
Envelope types (at least 2 fields: EnvelopeID (primary key) and Description, feel free to add categories, etc.)
Inventory (LocationID (foreign key), EnvelopeID (foreign key), Quantity and Date)
The ID fields are auto-numbered and will help when you do analyses, since numbers are easier to cross-reference than words.
Start with that, and once the data is in Access, we'll talk about querying the data