Let's look at how he signs off on the others.
Romans 16:20...The grace of our Lord Jesus be with you
1 cor 16:23 The grace of the Lord Jesus be with you.
2 Cor 13:13 The grace of the Lord Jesus Christ, and the love
a of God, and the fellowship of the Holy Spirit be with you all
Gal 5:18 Brothers and sisters, the grace of our Lord Jesus Christ be with your spirit
Eph 6:24 Grace be with all who have undying love for our Lord Jesus Christ.
Phil 4:23 The grace of the Lord Jesus Christ be with your spirit
Col 4:18 I, Paul, am writing this greeting with my own hand. Remember my chains. Grace be with you
1 Thess 5:28 The grace of our Lord Jesus Christ be with you.
2 Thess 3:16-18 The Lord be with all of you.
c 17I, Paul, am writing this greeting with my own hand, which is an authenticating mark in every letter; this is how I write.
18The grace of our Lord Jesus Christ be with you all
(Does he mean "The grace of our Lord be with you all" as the token/mark?)
1 Tim 6:21 Grace be with you all.
2 Tim 4:
22The Lord be with your spirit. Grace be with you all.
Titus 3: 15 Grace be with all of you.
Phm 1:25 The grace of the Lord Jesus Christ be with your spirit
Heb 13:25 Grace be with you all.
It appears that his method of signing off was the "Grace be with you all" or something close and for that reason along with other reasons. such as content that is similar in other letters, Hebrews is written by Paul and he signed off in that token that he was known for as he mentioned it in 2 Thess 3:16-18.