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You shouldn't have any problem since you sent it two days ahead. I think where most people have problems is when they send it on the last day or mail it too late to get there. Then they can also forget that there is a late fee, etc. and the problem compounds the next month. Just hold onto your Western Union receipt until you see the payment credited on the account. Or you could just scan it into your computer and keep a running tally of all your receipts in the event there ever is a discrepancy.
I have Santander right now. You'll be fine.
When you use the payment service to use a debit card I think they allow you to pay same day up until like 7pm or something like that.... I have had Santander in the past and have missed the payment by a week or more... and paid via the credit card thing like you did and it was just fine... didn't even have late fees or anything...
I have had 2 loans with them and I really have not had issues as many have... I even had a loan way back that went road loans to them and yes it was reported messed up but was able to get it fixed with a lady at santander.
if i have ever gotten a bad egg, that thought she could run the conversation and intimitate me I eventually realized she thought she could do as she wanted because she probably did with others and hung up and called back and got someone that knew the laws and it was fine... they are not bad... if you need them! (interest can be high)