If you plan on selling physical artwork, you'll need to familiarize yourself with some supplies. I'll talk about the different types of packaging needed for different types of art!

Small Flat Artwork

Rigid Envelopes
I stay stocked up on size A2 (13.25 x 9.25" because my most popular painting size is 9"x12"
They fit perfectly in these envelopes!
I buy mine in bulk on Amazon so they cost less than $1.20 each (free delivery)

Thick Artwork (framed work & canvas)

There are many ways you can package your thicker work, but over time I discovered the most cost-effective way is to build my own boxes.
I start off with large "mirror boxes" and cut them down to the size I need.

If you buy them in bulk, they should cost $5 or less each. And depending on the size of item you're shipping, you may be able to make two shipments out of one large box.
The mirror boxes I use are telescopic double-walled cardboard, so they are extremely rigid and protect anything inside.
I also buy bulk bubble wrap and packing paper for extra padding.
Here's a video showing my process for boxing larger items:

This video is from last year, and I've started using thicker boxes & added more padding (bubble wrap) to this process, but you get the idea!

The largest painting I've ever shipped was 2ft x 4ft, and I shipped it from Colorado to UK using this method. It cost $150 for 3 day Priority Express, Tracked, Insured mail.

Cost of Shipping - Domestic within US

Depending on the size of your art, you can save money by shipping Priority. If you ship flat artwork smaller than 12" on it's longest edge, you can use Priority envelopes and ship within the US for about $6.50 - Tracked & Insured
This is a great option if you want that guaranteed service.
For large artwork in thicker packages/boxes, expect to pay minimum $50 per shipment. This will include insurance, tracking, and guaranteed delivery (which your clients will expect). For canvas, my average cost within the US was $60.

Cost of Shipping - International

If you're shipping from within the US, to another country, the prices rise drastically.
For Priority and generic 12" envelopes, to most countries, you'll pay about $30 (Insured up to $50, but only tracked to the US border)
For larger packages/boxes you have a couple options. I ALWAYS ship Priority Express international because the risk of the item getting delayed or damaged is higher.
This costs about $80-150 to most countries (depending on size/weight of box).
This should be paid by the client.
If you're shipping from EU to a foreign country, the price is slightly better, especially for smaller flat artwork in rigid envelopes (anywhere from $10-30).
For larger packages/boxes within the EU expect to pay $50-100 (depending on size/weight of box)

Customs Forms

If you're shipping from the US to another country, you'll need to fill out a customs form for anything larger than a regular letter envelope. Such a pain!
The USPS Recently changed their customs forms so that all size packages use the same one (easier!). Grab a handful next time you're at the post office and fill them out ahead of time.
If you're shipping International Express, you need to get the customs form with the Express Mail EMS logo on it! Ask the clerk for these
Fill out the information as requested on the form, sign & date it.
If you're shipping from within the EU to an EU country, most times you won't need a customs form, unless it's a larger package, going outside the EU. Yay!

Personal Advice

I've shipped over 150 packages, most of them international.
I've only had ONE incident where a canvas was damaged during shipment (from Colorado to Denmark). There was a giant hole in the box and corner of the frame was broken, the canvas was slightly torn. My client was super OK with it, because he ended up framing it under glass, which flattened the canvas and you couldn't see the tear. I still refunded the shipping.

Besides that one incident, I've never had anything get damaged during shipping.
I still buy insurance for all my packages, just in case. It's all worked into the cost of shipping that the client pays.
It's a necessary precaution, in my opinion!

If you have any advice or stories of your own, please share them in the comments below!