Oddly, you didn’t say but let me assume you mean USA to USA.
By far the best way is USPS registered mail. Insurance is super cheap if registered and due to accountability it’s VERY secure.
Only downside is it can be slower but so what?
Added cost (above basic mailing charge) is about 13.00 then add up to 25,000.00 insurance at the rates below. This is an old charge list so it’s not accurate but at least it’ll give you an idea and you can search for up to date costs.
Registered Mail Prices
Declared Value Price
$0.00 $10.00
$0.01 to $100 $10.80
$100.01 to $500 $12.10
$500.01 to $1,000 $13.30
$1,000.01 to $2,000 $14.50
$2,000.01 to $3,000 $15.70
$3,000.01 to $4,000 $16.90
$4,000.01 to $5,000 $18.10
$5,000.01 to $6,000 $19.30
$6,000.01 to $7,000 $20.50
$7,000.01 to $8,000 $21.70
$8,000.01 to $9,000 $22.90
$9,000.01 to $10,000 $24.10
$10,000.01 to $11,000 $25.30
$11,000.01 to $12,000 $26.50
$12,000.01 to $13,000 $27.70
$13,000.01 to $14,000 $28.90
$14,000.01 to $15,000 $30.10
$15,000.01 to $16,000 $31.30
$16,000.01 to $17,000 $32.50
$17,000.01 to $18,000 $33.70
$18,000.01 to $19,000 $34.90
$19,000.01 to $20,000 $36.10
$20,000.01 to $21,000 $37.30
$21,000.01 to $22,000 $38.50
$22,000.01 to $23,000 $39.70
$23,000.01 to $24,000 $40.90
$24,000.01 to $25,000 $42.10
Keep in mind, you have to use paper tape only on ALL the seams.
Hope this helped