How to run AWS SDK with credentials from variables?

Try re-ordering your args so that ACCESS_KEY is the 1st param and SECRET_KEY is the second:

creds := credentials.NewStaticCredentials(conf.AWS_ACCESS_KEY_ID, conf.AWS_SECRET_ACCESS_KEY, "")

Try adding the region as well:

sess, err := session.NewSession(&aws.Config{
    Region:      aws.String("us-west-2"),
    Credentials: credentials.NewStaticCredentials(conf.AWS_ACCESS_KEY_ID, conf.AWS_SECRET_ACCESS_KEY, ""),
})

Additionally, if you hadn't known, the SDK allows for the use of the shared config under .aws/config. You can put your values in there and then set the environment variable AWS_SDK_LOAD_CONFIG to a truthy value to load the shared config. An example shared config would look like this:

[default]
aws_access_key_id = AKID
aws_secret_access_key = SECRET

Then running:

AWS_SDK_LOAD_CONFIG=true go run main.go

Or you can just temporaly set Environment variables.

package main
import (
    "fmt"
    "os"
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/s3/s3manager"
)

const (
    AccessKeyId     = "XXXXXXXXXXXXXXXXXX"
    SecretAccessKey = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
    Region          = "eu-west-1"
    Bucket          = "XXXXX-XXXX-XXX"
)

func main() {
    os.Setenv("AWS_ACCESS_KEY_ID",     AccessKeyId)
    os.Setenv("AWS_SECRET_ACCESS_KEY", SecretAccessKey)

    filename := os.Args[1]

    file, err := os.Open(filename)
    if err != nil {
        fmt.Println("Failed to open file", filename, err)
        os.Exit(1)
    }
    defer file.Close()

    conf := aws.Config{Region: aws.String(Region)}
    sess := session.New(&conf)

    svc := s3manager.NewUploader(sess)

    fmt.Println("Uploading file to S3...")
    result, err := svc.Upload(&s3manager.UploadInput{
        Bucket: aws.String(Bucket),
        Key:    aws.String(filepath.Base(filename)),
        Body:   file,
    })
    if err != nil {
        fmt.Println("error", err)
        os.Exit(1)
    }
}