No. You don’t have to send the original affidavit of support by mail.
You will find the following information in the interview letter:
If a financial sponsor submitted an I-864 Affidavit of Support form, you must give the consular officer a signed version. It can be a photocopy, but the signature cannot be typed or electronic.
There is also a video in YouTube posted last week by the U.S. embassy in Bogotá where they talk about this topic.
@Crazy Cat will stop by and give you a comparison list but what it boils down to is whether she wants (or needs to work) right away of if she can be unemployed for maybe over a year,
IMO, marry and go the CR1 route. The timing recently between K1 and CR1 is mostly about the same and CR1 just seems superior. Marriage will be accepted by USCIS if it is done legally where it took place.
Every couple has their own priorities, and each couple must decide which visa is better for their situation.
K-1
More expensive than CR-1
Requires Adjustment of Status after marriage (expensive and requires a lot of paperwork)
Spouse can not leave the US until she/he receives approved Advance Parole (approx 6-8 months)
Spouse can not work until she/he receives EAD (approx 6-8 months)
Some people have had problems with driver licenses, Social Security cards, leases, bank account during this period
Spouse will not receive Green Card for many months after Adjustment of Status is filed.
A K-1 might be a better choice when 18-21 year old children are immigrating also
In some situations, marriage can affect certain Home country benefits, making a K-1 a better choice
A denied K-1 is sent back to USCIS to expire
CR-1
Less expensive than K-1
No Adjustment of Status(I-485, I-131, I-765) required.
Spouse can immediately travel outside the US
Spouse is authorized to work immediately upon arrival.
Spouse receives Social Security Card and Green Card within 2 or 3 weeks after entering the US
Opening a bank account, getting a driver's license, etc. are very easily accomplished with GC, SS card, and passport.
Spouse has legal permanent Resident status IMMEDIATELY upon entry to US.